web: harden drop-open and autosave after adversarial review

Review of the drop-open and autosave commits surfaced one blocker and
five majors; all are fixed here, and two claims in the autosave commit
message are corrected by this one: tab close does NOT clear a snapshot
(matches native .sv$ semantics — saving, restoring, discarding, exit,
and the capped boot scan do), and serialization runs on the main
thread between awaits, not off the paint path.

- The restore prompt can no longer appear with no entries (empty
  "Recover Drawing" modal on every boot once the autosave dir exists).
- Restore no longer consumes the snapshot before the open succeeds:
  the recovered drawing goes through the guarded drop queue, and the
  snapshot is deleted only after the tab opens; cancel or parse
  failure keeps the only copy.
- Saving during an autosave round defers its snapshot cleanup until
  the round finishes, so a stale round can no longer resurrect a
  snapshot newer than the save.
- The boot scan no longer deletes snapshots by mtime comparison
  (re-opening a drawing refreshes the recent copy's mtime, which
  silently destroyed wanted snapshots); GC is now a 16-entry cap with
  oldest-first eviction.
- OPFS write failures abort the writable stream (a leaked lock could
  stall the same-key writer and disable autosave for the session), and
  DOMException errors now carry their name/message.
- Empty (crash-window) snapshots are never offered; an unreadable one
  warns and stays discardable instead of looping forever.
- Drop-open polish: reads start inside the handler and complete in
  drop order (legacy WebKit safety, deterministic multi-file order),
  dragenter is canceled too, the install guard only latches on
  success, non-drawing drops don't steal propagation, a lock-poison
  panic can't cross into JS, and re-dropping an open drawing no
  longer risks clobbering an in-flight open (it re-queues).
- The model-space sheet floor is per-axis (a thin 8x400 mm profile
  keeps its length) and centers content on the enlarged page.

src/app/mod.rs

@@ -736,6 +736,16 @@ pub(super) struct OpenCADStudio {
     /// prompt still has to offer, one at a time.
     #[cfg(target_arch = "wasm32")]
     web_autosave_pending: Vec<crate::io::web_autosave::WebAutosaveEntry>,
+    /// The snapshot whose recovered open is queued: `(autosave key,
+    /// open name)`. The OPFS copy is the only one, so it is deleted only
+    /// once an open with exactly that name succeeds.
+    #[cfg(target_arch = "wasm32")]
+    web_autosave_consumed: Option<(String, String)>,
+    /// Autosave keys a web save wanted to remove while a snapshot round was
+    /// still writing (the round's late write would resurrect the snapshot
+    /// with a fresh mtime); the removes run when the round finishes.
+    #[cfg(target_arch = "wasm32")]
+    web_autosave_clear_after: Vec<String>,
     // ── Floating panel windows ────────────────────────────────────────────
     /// Active `iced_aw` colour picker: destination plus its initial colour.
     color_pick_target: Option<(ColorPickTarget, AcadColor)>,
@@ -2021,8 +2031,8 @@ pub enum Message {
     /// Web: delete the offered autosave snapshot (and offer the next, if any).
     #[cfg(target_arch = "wasm32")]
     WebAutosaveDiscard,
-    /// Web: the offered snapshot's bytes are in (and the OPFS copy consumed);
+    /// Web: the offered snapshot's bytes are in; queue them for the guarded
-    /// open them as `name`.
+    /// open poll. The OPFS copy is consumed only once that open succeeds.
     #[cfg(target_arch = "wasm32")]
     WebAutosaveBytes(String, Result<Vec<u8>, String>),
     /// Web: one autosave round finished for all dirty tabs (serially).
@@ -3769,6 +3779,10 @@ impl OpenCADStudio {
             web_autosave_warned: false,
             #[cfg(target_arch = "wasm32")]
             web_autosave_pending: Vec::new(),
+            #[cfg(target_arch = "wasm32")]
+            web_autosave_consumed: None,
+            #[cfg(target_arch = "wasm32")]
+            web_autosave_clear_after: Vec::new(),
             color_pick_target: None,
             color_picker_tab: ColorPickerTab::Index,
             recent_colors: Vec::new(),

src/app/startup.rs

@@ -26,14 +26,21 @@ impl OpenCADStudio {
 
     /// Queue the web crash-recovery prompt behind whatever startup modal is
     /// already showing (donation, GPU warning…); the after-message hook in
-    /// `update` surfaces it as soon as the slot is free.
+    /// `update` surfaces it as soon as the slot is free. No-op when nothing
+    /// is pending: once the autosave OPFS directory merely exists (even with
+    /// no entries), queueing unconditionally would show an empty
+    /// "Recover Drawing" modal on every boot.
     #[cfg(target_arch = "wasm32")]
     pub(super) fn queue_autosave_restore_prompt(&mut self) {
+        if self.web_autosave_pending.is_empty() {
+            return;
+        }
         if !self
             .pending_startup_modals
             .contains(&ModalKind::AutosaveRestore)
         {
-            self.pending_startup_modals.push_back(ModalKind::AutosaveRestore);
+            self.pending_startup_modals
+                .push_back(ModalKind::AutosaveRestore);
         }
         self.show_next_startup_modal();
     }
@@ -62,7 +69,8 @@ impl OpenCADStudio {
         };
         match &self.gpu_status {
             GpuStatus::Software(adapter) => {
-                self.command_line.push_warning(&software_rasterizer_warning(adapter));
+                self.command_line
+                    .push_warning(&software_rasterizer_warning(adapter));
             }
             GpuStatus::NoRenderer => {
                 crate::scene::pipeline::report_gpu_line(
@@ -141,9 +149,15 @@ mod tests {
         // The name carries parentheses and commas of its own; the localized
         // template must still place it whole. The loader follows the system
         // locale here, so the assertions stay language-neutral.
-        assert!(warning.contains("(llvmpipe (LLVM 20.1.8, 256 bits))"), "{warning}");
+        assert!(
+            warning.contains("(llvmpipe (LLVM 20.1.8, 256 bits))"),
+            "{warning}"
+        );
         assert!(warning.contains("GPU"), "{warning}");
-        assert!(!warning.contains("{adapter}") && !warning.contains("__ocs_"), "{warning}");
+        assert!(
+            !warning.contains("{adapter}") && !warning.contains("__ocs_"),
+            "{warning}"
+        );
         // Every platform has a hint, and it is a sentence, not a missing key.
         let hint = gpu_platform_hint();
         assert!(hint.len() > 20 && !hint.contains("hint-"), "{hint}");

src/app/update/file.rs

@@ -1936,22 +1936,33 @@ impl OpenCADStudio {
                 );
                 // A clean save supersedes any crash-recovery snapshot for this
                 // drawing — drop the old key and the new one so stale copies
-                // don't resurface in the next boot scan.
+                // don't resurface in the next boot scan. But an autosave
+                // round may still be writing the same key: its write would
+                // land AFTER the remove and resurrect the snapshot (with a
+                // newer mtime, so the next scan would offer it as
+                // recoverable). Defer the removes to the end of the round.
                 let new_key = self.tabs[i]
                     .current_path
                     .as_ref()
                     .map(|path| path.to_string_lossy().into_owned())
                     .unwrap_or_default();
-                recent_task = Task::batch([
+                if self.web_autosave_in_flight {
-                    recent_task,
+                    self.web_autosave_clear_after.push(autosave_key);
-                    Task::perform(
+                    if !new_key.is_empty() {
-                        async move {
+                        self.web_autosave_clear_after.push(new_key);
-                            let _ = crate::io::web_autosave::remove(&autosave_key).await;
+                    }
-                            let _ = crate::io::web_autosave::remove(&new_key).await;
+                } else {
-                        },
+                    recent_task = Task::batch([
-                        |_| Message::Noop,
+                        recent_task,
-                    ),
+                        Task::perform(
-                ]);
+                            async move {
+                                let _ = crate::io::web_autosave::remove(&autosave_key).await;
+                                let _ = crate::io::web_autosave::remove(&new_key).await;
+                            },
+                            |_| Message::Noop,
+                        ),
+                    ]);
+                }
                 self.command_line
                     .push_output(crate::tf!("Saved: {filename}").as_ref());
                 true
@@ -2964,10 +2975,21 @@ impl OpenCADStudio {
     }
 
     /// One web autosave round finished. Silent on success; on failure one
-    /// command-line warning until a round succeeds again.
+    /// command-line warning until a round succeeds again. Also the drain
+    /// point for snapshot removes a save deferred while the round was
+    /// writing: the round's writes are done, so the keys can be cleared
+    /// for real now (no late write can resurrect them).
     #[cfg(target_arch = "wasm32")]
     pub(super) fn on_web_autosave_finished(&mut self, result: Result<(), String>) -> Task<Message> {
         self.web_autosave_in_flight = false;
+        let deferred = std::mem::take(&mut self.web_autosave_clear_after);
+        if !deferred.is_empty() {
+            wasm_bindgen_futures::spawn_local(async move {
+                for key in &deferred {
+                    let _ = crate::io::web_autosave::remove(key).await;
+                }
+            });
+        }
         match result {
             Ok(()) => self.web_autosave_warned = false,
             Err(error) => {
@@ -2983,7 +3005,10 @@ impl OpenCADStudio {
     }
 
     /// The boot scan decided which snapshots are recoverable; queue the
-    /// restore prompt (behind any already-queued startup modal).
+    /// restore prompt (behind any already-queued startup modal). An empty
+    /// scan — a fresh profile, or everything already cleaned up — must not
+    /// surface an empty "Recover Drawing" modal, so the prompt is only
+    /// queued when there is something to offer.
     #[cfg(target_arch = "wasm32")]
     pub(super) fn on_web_autosaves_scanned(
         &mut self,
@@ -2999,8 +3024,12 @@ impl OpenCADStudio {
         Task::none()
     }
 
-    /// Restore the offered snapshot: read (and consume) the OPFS copy, then
+    /// Restore the offered snapshot: read the OPFS copy — leaving it in
-    /// open it as a recovered tab through the standard web-bytes open flow.
+    /// place — and hand the bytes to the 50 ms poll's guarded open queue.
+    /// The snapshot is the only copy, so it is deleted only once the
+    /// recovered drawing actually opens (`web_autosave_opened`, on the
+    /// matching `WebFileOpened` success); a parse failure or cancel leaves
+    /// it recoverable for the next boot.
     #[cfg(target_arch = "wasm32")]
     pub(super) fn on_web_autosave_restore(&mut self) -> Task<Message> {
         let Some(entry) = self.web_autosave_pending.first().cloned() else {
@@ -3011,18 +3040,37 @@ impl OpenCADStudio {
         self.close_active_modal();
         let open_name = crate::io::web_autosave::recovered_name(&entry.name);
         let key = entry.name;
+        // Remember the pair: `web_autosave_opened` deletes the OPFS copy
+        // only when an open with exactly `open_name` succeeds. A failed
+        // read below clears the marker again (`on_web_autosave_bytes`).
+        self.web_autosave_consumed = Some((key.clone(), open_name.clone()));
         Task::perform(
-            async move {
+            async move { crate::io::web_autosave::read(&key).await },
-                let bytes = crate::io::web_autosave::read(&key).await?;
-                // The snapshot is spent: the recovered tab (and its recent
-                // copy) replaces it from here on.
-                let _ = crate::io::web_autosave::remove(&key).await;
-                Ok(bytes)
-            },
             move |result| Message::WebAutosaveBytes(open_name, result),
         )
     }
 
+    /// The recovered drawing opened: its autosave snapshot — held back until
+    /// now — can finally be deleted, and the next queued snapshot can be
+    /// offered. A different name finishing first just means the recovered
+    /// open is still queued behind it; keep waiting.
+    #[cfg(target_arch = "wasm32")]
+    pub(super) fn web_autosave_opened(&mut self, open_name: &str) {
+        let Some((key, recorded)) = self.web_autosave_consumed.clone() else {
+            return;
+        };
+        if recorded != open_name {
+            return;
+        }
+        self.web_autosave_consumed = None;
+        self.command_line
+            .push_output(crate::tf!("Restored \"{}\" from the autosave copy", open_name).as_ref());
+        wasm_bindgen_futures::spawn_local(async move {
+            let _ = crate::io::web_autosave::remove(&key).await;
+        });
+        self.queue_next_web_autosave_prompt();
+    }
+
     /// Discard the offered snapshot; the prompt stays up for the next one.
     #[cfg(target_arch = "wasm32")]
     pub(super) fn on_web_autosave_discard(&mut self) -> Task<Message> {
@@ -3038,8 +3086,13 @@ impl OpenCADStudio {
         Task::none()
     }
 
-    /// The offered snapshot's bytes arrived (or did not): open the recovered
+    /// The offered snapshot's bytes arrived (or did not). On success the
-    /// tab, or warn and move on to the next snapshot.
+    /// bytes ride the 50 ms poll's guarded open queue — the poll starts an
+    /// open only when the slot is free, so the recovered drawing can
+    /// neither clobber an open in flight nor be lost to one, and the
+    /// snapshot survives until that open lands (`web_autosave_opened`).
+    /// On failure the snapshot stays in OPFS (discardable on a later boot)
+    /// and the prompt moves on to the next one.
     #[cfg(target_arch = "wasm32")]
     pub(super) fn on_web_autosave_bytes(
         &mut self,
@@ -3048,31 +3101,33 @@ impl OpenCADStudio {
     ) -> Task<Message> {
         match result {
             Ok(bytes) => {
-                self.command_line.push_output(
+                // The next prompt is queued only when this open settles
-                    crate::tf!("Restored \"{}\" from the autosave copy", open_name).as_ref(),
+                // (`web_autosave_opened` / the failure hook), not here — the
-                );
+                // poll has not started the open yet, so queueing now would
-                let task = self.update(Message::WebFileDropped(open_name, bytes));
+                // show the modal before the recovered drawing loads.
-                // Further snapshots wait behind this one; queue the prompt
+                crate::io::web_drop::enqueue_open(open_name, bytes);
-                // again now that an open is in flight, so it reappears when
-                // the recovered drawing finishes opening.
-                if !self.web_autosave_pending.is_empty()
-                    && !self
-                        .pending_startup_modals
-                        .contains(&crate::app::ModalKind::AutosaveRestore)
-                {
-                    self.pending_startup_modals
-                        .push_back(crate::app::ModalKind::AutosaveRestore);
-                }
-                task
-            }
-            Err(error) => {
-                self.command_line
-                    .push_warning(crate::tf!("Autosave copy could not be read: {error}").as_ref());
-                if !self.web_autosave_pending.is_empty() {
-                    self.queue_autosave_restore_prompt();
-                }
                 Task::none()
             }
+            Err(error) => {
+                // Unreadable snapshot: warn, but neither delete it nor
+                // re-offer it this session — it was already taken out of
+                // `web_autosave_pending`, so the cycle cannot loop on it.
+                // It stays discardable on the next boot.
+                self.web_autosave_consumed = None;
+                self.command_line
+                    .push_warning(crate::tf!("Autosave copy could not be read: {error}").as_ref());
+                self.queue_next_web_autosave_prompt();
+                Task::none()
+            }
+        }
+    }
+
+    /// Re-offer the remaining boot snapshots once the current one settled.
+    /// No-op when none are left (or the modal is already queued).
+    #[cfg(target_arch = "wasm32")]
+    pub(super) fn queue_next_web_autosave_prompt(&mut self) {
+        if !self.web_autosave_pending.is_empty() {
+            self.queue_autosave_restore_prompt();
         }
     }
 
@@ -3723,8 +3778,18 @@ impl OpenCADStudio {
                 let width = ((max.x - min.x) as f64 * margin).max(1.0);
                 let height = ((max.y - min.y) as f64 * margin).max(1.0);
                 if width < MIN_MODEL_SHEET_MM || height < MIN_MODEL_SHEET_MM {
+                    // Per-axis: a thin-but-long profile keeps its long
+                    // dimension instead of being clipped to the sheet, and
+                    // the content is centered on the enlarged page.
                     let (w, h) = standard_sheet.unwrap_or((297.0, 210.0));
-                    (w, h, -min.x as f64, -min.y as f64)
+                    let page_w = width.max(w);
+                    let page_h = height.max(h);
+                    (
+                        page_w,
+                        page_h,
+                        (page_w - width) * 0.5 - min.x as f64,
+                        (page_h - height) * 0.5 - min.y as f64,
+                    )
                 } else {
                     let pad_x = (width - (max.x - min.x) as f64) * 0.5;
                     let pad_y = (height - (max.y - min.y) as f64) * 0.5;

src/app/update/mod.rs

@@ -811,6 +811,13 @@ impl OpenCADStudio {
                     self.command_line
                         .push_info(crate::tf!("Open cancelled: \"{}\"", p.name).as_ref());
                 }
+                // A cancelled recovered open keeps its snapshot (re-offered
+                // next boot — it is the only copy); the remaining queued
+                // snapshots can still be offered this session.
+                #[cfg(target_arch = "wasm32")]
+                if self.web_autosave_consumed.take().is_some() {
+                    self.queue_next_web_autosave_prompt();
+                }
                 self.drain_pending_open()
             }
 
@@ -818,7 +825,13 @@ impl OpenCADStudio {
             Message::WebFileDropped(name, bytes) => {
                 // Bytes are already in hand (no picker round-trip), so this is
                 // `on_open_file`'s web branch with the real name and size on
-                // the overlay from the start.
+                // the overlay from the start. An open already in flight must
+                // not be clobbered (its bytes are gone once dequeued), so the
+                // pair goes back to the queue for the next poll tick.
+                if self.opening.is_some() {
+                    crate::io::web_drop::enqueue_open(name, bytes);
+                    return Task::none();
+                }
                 let state = std::sync::Arc::new(crate::io::OpenProgressState::new(
                     crate::app::OPEN_PHASE_READING,
                 ));
@@ -845,6 +858,22 @@ impl OpenCADStudio {
                 if self.opening.as_ref().map(|opening| opening.id) != Some(open_id) {
                     return Task::none();
                 }
+                // A crash-recovery open settling here: success deletes its
+                // OPFS snapshot (the only copy, kept until now); a parse
+                // failure keeps it for the next boot. Either way the next
+                // queued snapshot can be offered again.
+                if self
+                    .web_autosave_consumed
+                    .as_ref()
+                    .is_some_and(|(_, recorded)| *recorded == outcome.name)
+                {
+                    if outcome.result.is_ok() {
+                        self.web_autosave_opened(&outcome.name);
+                    } else {
+                        self.web_autosave_consumed = None;
+                        self.queue_next_web_autosave_prompt();
+                    }
+                }
                 if let Some(opening) = self.opening.as_mut() {
                     opening.name = outcome.name.clone();
                     opening.source_path = Some(std::path::PathBuf::from(&outcome.name));
@@ -4455,8 +4484,7 @@ impl OpenCADStudio {
                 Task::none()
             }
             Message::AutoConstrainReset => {
-                self.auto_constrain_settings =
+                self.auto_constrain_settings = super::settings::AutoConstrainSettings::default();
-                    super::settings::AutoConstrainSettings::default();
                 self.auto_constrain_selected_row = 0;
                 self.auto_constrain_distance_input =
                     format!("{}", self.auto_constrain_settings.distance_tolerance);
@@ -4495,8 +4523,7 @@ impl OpenCADStudio {
                         self.auto_constrain_settings.distance_tolerance = distance;
                         self.auto_constrain_settings.angle_tolerance_deg = angle;
                         self.auto_constrain_settings.sanitize();
-                        self.auto_constrain_saved =
+                        self.auto_constrain_saved = Some(self.auto_constrain_settings.clone());
-                            Some(self.auto_constrain_settings.clone());
                         self.persist_settings_if_changed();
                         if matches!(action, Message::AutoConstrainOk) {
                             self.close_active_modal();

src/app/view/mod.rs

@@ -832,17 +832,19 @@ bg={bg_ms:.1}ms n={view_count}"
                         self.constraint_bar_display,
                     )
                     .into_iter()
-                    .map(|(id, point, direction, label, is_conflicting, hover_points)| {
+                    .map(
-                        let selected = tab.scene.selected_constraint == Some(id);
+                        |(id, point, direction, label, is_conflicting, hover_points)| {
-                        (
+                            let selected = tab.scene.selected_constraint == Some(id);
-                            point,
+                            (
-                            direction,
+                                point,
-                            label,
+                                direction,
-                            is_conflicting,
+                                label,
-                            selected,
+                                is_conflicting,
-                            hover_points,
+                                selected,
-                        )
+                                hover_points,
-                    })
+                            )
+                        },
+                    )
                     .collect()
             };
             crate::ui::overlay::selection_overlay(

src/io/web_autosave.rs

@@ -8,11 +8,18 @@
 //! offered for restore at boot when newer than the drawing's last explicit
 //! save (the OPFS recent copy of the same name).
 //!
+//! Losing that comparison never deletes a snapshot: the recent copy is
+//! refreshed by every OPEN as well as every save, so a snapshot that just
+//! went stale may still be the only survivor of a crash right after a
+//! re-open. Accumulation is bounded instead — the boot scan keeps the
+//! newest [`SNAPSHOT_CAP`] snapshots and evicts only the oldest ones beyond
+//! that cap.
+//!
 //! OPFS entry names carry only the FNV hash of the drawing name (like
 //! `web_recent`), so a small JSON index maps hashes back to names. The index
 //! is written before its snapshot: a crash between the two writes leaves a
-//! name without bytes, which `list` filters out, rather than an orphan
+//! name without bytes — a missing file or a zero-byte one — which `list`
-//! snapshot nobody can name.
+//! filters out, rather than an orphan snapshot nobody can name.
 
 #[cfg(target_arch = "wasm32")]
 use wasm_bindgen::JsCast;
@@ -38,6 +45,9 @@ pub struct WebAutosaveEntry {
 /// A snapshot is worth offering when it is strictly newer than the last
 /// explicit save, or when no saved copy exists at all (the drawing was never
 /// saved — or its recent copy was evicted — so the snapshot is all there is).
+/// The comparison decides *offering* only, never survival: the recent copy
+/// is refreshed by every open, so "older than the recent copy" is not proof
+/// that a snapshot recovers nothing.
 #[cfg_attr(not(target_arch = "wasm32"), allow(dead_code))] // unit-tested on native
 pub(crate) fn newer_than_saved(autosave_ms: f64, saved_ms: Option<f64>) -> bool {
     match saved_ms {
@@ -46,6 +56,33 @@ pub(crate) fn newer_than_saved(autosave_ms: f64, saved_ms: Option<f64>) -> bool
     }
 }
 
+/// Upper bound on stored snapshots; the boot scan evicts the oldest ones
+/// beyond it. Without a bound, snapshots superseded by a save (or by a
+/// mere re-open) would pile up forever, since age no longer deletes them.
+#[cfg_attr(not(target_arch = "wasm32"), allow(dead_code))] // unit-tested on native
+pub(crate) const SNAPSHOT_CAP: usize = 16;
+
+/// Split `entries` into `(kept, evicted)` so at most `cap` snapshots remain,
+/// both sides ordered by `modified_ms` (newest first within each side; the
+/// evicted names overall run oldest-first, so an interrupted scan deletes
+/// the least valuable snapshots first). This is the only deletion criterion
+/// the boot scan applies — being older than the recent copy is not one.
+#[cfg_attr(not(target_arch = "wasm32"), allow(dead_code))] // unit-tested on native
+pub(crate) fn cap_evictions(
+    mut entries: Vec<WebAutosaveEntry>,
+    cap: usize,
+) -> (Vec<WebAutosaveEntry>, Vec<String>) {
+    entries.sort_by(|a, b| b.modified_ms.total_cmp(&a.modified_ms));
+    let overflow = entries.len().saturating_sub(cap);
+    let mut evicted: Vec<String> = entries
+        .split_off(entries.len() - overflow)
+        .into_iter()
+        .map(|entry| entry.name)
+        .collect();
+    evicted.reverse();
+    (entries, evicted)
+}
+
 /// The name a restored drawing carries as a tab and download: the drawing's
 /// stem plus " (recovered)" and its own extension (DWG when the stored name
 /// has none, matching how autosave serializes untitled tabs). The extension
@@ -89,7 +126,10 @@ pub async fn snapshot(name: &str, bytes: &[u8], thumbnail: Option<&[u8]>) -> Res
 }
 
 /// All live snapshots. Index rows whose file is gone (crash between writes,
-/// partial eviction) are skipped rather than reported.
+/// partial eviction) and zero-byte snapshots (the crash window after the
+/// index write but before the snapshot write) are skipped rather than
+/// reported — an empty file recovers nothing and would only route the user
+/// into the corrupt-file recovery flow.
 #[cfg(target_arch = "wasm32")]
 pub async fn list() -> Result<Vec<WebAutosaveEntry>, String> {
     let directory = autosave_directory(false).await?;
@@ -98,6 +138,9 @@ pub async fn list() -> Result<Vec<WebAutosaveEntry>, String> {
         let Ok(file) = super::web_recent::get_file(&directory, &snapshot_key(&name)).await else {
             continue;
         };
+        if file.size() == 0.0 {
+            continue;
+        }
         entries.push(WebAutosaveEntry {
             name,
             size: file.size() as u64,
@@ -130,22 +173,26 @@ pub async fn remove(name: &str) -> Result<(), String> {
     drawing.map(|_| ())
 }
 
-/// Boot scan: keep the snapshots worth offering (newer than the drawing's
+/// Boot scan: offer the snapshots worth recovering — strictly newer than the
-/// last explicit save, newest first) and silently delete the stale ones — a
+/// drawing's last explicit copy, newest first. Age never deletes: the recent
-/// snapshot older than the saved copy recovers nothing and would otherwise
+/// copy is refreshed by every OPEN as well as every save, so a snapshot that
-/// pile up across sessions.
+/// lost the comparison may still be the only survivor of a crash right after
+/// a re-open. Accumulation is bounded instead — beyond [`SNAPSHOT_CAP`]
+/// snapshots the oldest are evicted, which is the only deletion here.
 #[cfg(target_arch = "wasm32")]
 pub async fn scan() -> Result<Vec<WebAutosaveEntry>, String> {
+    let (kept, evicted) = cap_evictions(list().await?, SNAPSHOT_CAP);
+    for name in evicted {
+        let _ = remove(&name).await;
+    }
     let mut recoverable = Vec::new();
-    for entry in list().await? {
+    for entry in kept {
         let saved_ms = super::web_recent::modified_ms(&entry.name).await;
         if newer_than_saved(entry.modified_ms, saved_ms) {
             recoverable.push(entry);
-        } else {
-            let _ = remove(&entry.name).await;
         }
     }
-    recoverable.sort_by(|a, b| b.modified_ms.total_cmp(&a.modified_ms));
+    // `cap_evictions` already ordered the survivors newest-first.
     Ok(recoverable)
 }
 
@@ -219,4 +266,44 @@ mod tests {
         assert_eq!(recovered_name("a.b.dwg"), "a.b (recovered).dwg");
         assert_eq!(recovered_name(""), " (recovered).dwg");
     }
+
+    #[test]
+    fn under_the_cap_nothing_is_evicted_and_order_is_newest_first() {
+        let (kept, evicted) = cap_evictions(
+            vec![
+                entry("old.dwg", 100.0),
+                entry("new.dwg", 300.0),
+                entry("mid.dwg", 200.0),
+            ],
+            SNAPSHOT_CAP,
+        );
+        assert!(evicted.is_empty());
+        assert_eq!(
+            kept.iter().map(|e| e.name.as_str()).collect::<Vec<_>>(),
+            ["new.dwg", "mid.dwg", "old.dwg"]
+        );
+    }
+
+    #[test]
+    fn over_the_cap_exactly_the_oldest_overflow_is_evicted() {
+        let entries = (0..SNAPSHOT_CAP + 4)
+            .map(|i| entry(&format!("d{i}.dwg"), i as f64))
+            .collect();
+        let (kept, evicted) = cap_evictions(entries, SNAPSHOT_CAP);
+        assert_eq!(kept.len(), SNAPSHOT_CAP);
+        // Newest kept first, oldest evicted first.
+        assert_eq!(kept.first().map(|e| e.name.as_str()), Some("d19.dwg"));
+        assert_eq!(
+            evicted.iter().map(|name| name.as_str()).collect::<Vec<_>>(),
+            ["d0.dwg", "d1.dwg", "d2.dwg", "d3.dwg"]
+        );
+    }
+
+    fn entry(name: &str, modified_ms: f64) -> WebAutosaveEntry {
+        WebAutosaveEntry {
+            name: name.to_string(),
+            size: 16,
+            modified_ms,
+        }
+    }
 }

src/io/web_drop.rs

@@ -5,7 +5,8 @@
 //! queues them for the 50 ms control poll to open through the web-picker flow.
 
 use std::collections::VecDeque;
-use std::sync::{Mutex, OnceLock};
+use std::sync::atomic::{AtomicBool, Ordering};
+use std::sync::Mutex;
 use wasm_bindgen::JsCast;
 use wasm_bindgen_futures::JsFuture;
 
@@ -14,14 +15,25 @@ use wasm_bindgen_futures::JsFuture;
 const QUEUE_CAP: usize = 8;
 
 static DROPPED: Mutex<VecDeque<(String, Vec<u8>)>> = Mutex::new(VecDeque::new());
-static INSTALLED: OnceLock<()> = OnceLock::new();
+// `OnceLock::take` needs `&mut self`, which an immutable static cannot
+// provide — an AtomicBool swap gives the same set-once marker and can still
+// be reverted when no document exists.
+static INSTALLED: AtomicBool = AtomicBool::new(false);
 
 fn warn(line: &str) {
     web_sys::console::warn_1(&wasm_bindgen::JsValue::from_str(line));
 }
 
+fn lock_queue() -> std::sync::MutexGuard<'static, VecDeque<(String, Vec<u8>)>> {
+    // The critical sections are push/pop only, so poisoning is unreachable;
+    // recover anyway rather than panic across the JS boundary.
+    DROPPED
+        .lock()
+        .unwrap_or_else(|poisoned| poisoned.into_inner())
+}
+
 fn enqueue(name: String, bytes: Vec<u8>) {
-    let mut queue = DROPPED.lock().unwrap();
+    let mut queue = lock_queue();
     if queue.len() >= QUEUE_CAP {
         drop(queue);
         warn(&format!(
@@ -32,6 +44,12 @@ fn enqueue(name: String, bytes: Vec<u8>) {
     queue.push_back((name, bytes));
 }
 
+/// Queue already-read drawing bytes (drag-and-drop, autosave restore) for the
+/// 50 ms poll to open through the guarded web-picker flow.
+pub fn enqueue_open(name: String, bytes: Vec<u8>) {
+    enqueue(name, bytes);
+}
+
 /// JS-callable entry (same shape as `ocs_control_submit`) for hosts that
 /// forward drops themselves.
 #[wasm_bindgen::prelude::wasm_bindgen]
@@ -41,7 +59,7 @@ pub fn ocs_web_drop_file(name: String, bytes: js_sys::Uint8Array) {
 
 /// Pop the oldest dropped file that the poll has not started opening yet.
 pub fn take_dropped() -> Option<(String, Vec<u8>)> {
-    DROPPED.lock().unwrap().pop_front()
+    lock_queue().pop_front()
 }
 
 /// Feed already-read bytes through the same loader tail the web picker uses
@@ -71,35 +89,44 @@ pub async fn open_dropped(
     }
 }
 
-/// Register the document-level `dragover`/`drop` listeners once. `dragover`
+/// Register the document-level `dragenter`/`dragover`/`drop` listeners once.
-/// MUST preventDefault or the browser navigates to the dropped file instead
+/// `dragover` MUST preventDefault or the browser navigates to the dropped file
-/// of delivering it to the page.
+/// instead of delivering it to the page.
 pub fn install() {
-    if INSTALLED.set(()).is_err() {
+    if INSTALLED.swap(true, Ordering::SeqCst) {
         return;
     }
     let Some(document) = web_sys::window().and_then(|window| window.document()) else {
+        // Only mark installed once the listeners really are: an early
+        // no-document context must not permanently disable drops.
+        INSTALLED.store(false, Ordering::SeqCst);
         return;
     };
-    let on_dragover = wasm_bindgen::closure::Closure::<dyn FnMut(web_sys::Event)>::new(
+    let cancel = wasm_bindgen::closure::Closure::<dyn FnMut(web_sys::Event)>::new(
         move |event: web_sys::Event| {
             event.prevent_default();
         },
     );
-    let _ =
+    // Canceling `dragenter` too is belt-and-braces some engines still want
-        document.add_event_listener_with_callback("dragover", on_dragover.as_ref().unchecked_ref());
+    // before treating the page as a drop target.
-    on_dragover.forget();
+    let _ = document.add_event_listener_with_callback("dragenter", cancel.as_ref().unchecked_ref());
+    let _ = document.add_event_listener_with_callback("dragover", cancel.as_ref().unchecked_ref());
+    cancel.forget();
 
     let on_drop = wasm_bindgen::closure::Closure::<dyn FnMut(web_sys::Event)>::new(
         move |event: web_sys::Event| {
             event.prevent_default();
-            event.stop_propagation();
             let Some(drag) = event.dyn_ref::<web_sys::DragEvent>() else {
                 return;
             };
             let Some(files) = drag.data_transfer().and_then(|transfer| transfer.files()) else {
                 return;
             };
+            // Start every read here, inside the handler, and finish them in
+            // one task in drop order: legacy WebKit dislikes drag-originated
+            // reads that begin after the event returns, and parallel reads
+            // would enqueue (and open) in completion order, not drop order.
+            let mut reads: Vec<(String, js_sys::Promise)> = Vec::new();
             for index in 0..files.length() {
                 let Some(file) = files.get(index) else {
                     continue;
@@ -113,15 +140,27 @@ pub fn install() {
                     ));
                     continue;
                 }
-                wasm_bindgen_futures::spawn_local(async move {
+                reads.push((name, file.array_buffer()));
-                    match read_file(&file).await {
-                        Ok(bytes) => enqueue(name, bytes),
-                        Err(error) => web_sys::console::error_1(&wasm_bindgen::JsValue::from_str(
-                            &format!("ocs: failed to read dropped file \"{name}\": {error}"),
-                        )),
-                    }
-                });
             }
+            if reads.is_empty() {
+                return;
+            }
+            // The page is the only intended drop target; hide drawing drops
+            // from host-page handlers only once there is something to open.
+            event.stop_propagation();
+            wasm_bindgen_futures::spawn_local(async move {
+                for (name, read) in reads {
+                    match JsFuture::from(read).await {
+                        Ok(buffer) => enqueue(name, js_sys::Uint8Array::new(&buffer).to_vec()),
+                        Err(error) => {
+                            web_sys::console::error_1(&wasm_bindgen::JsValue::from_str(&format!(
+                                "ocs: failed to read dropped file \"{name}\": {}",
+                                error.as_string().unwrap_or_else(|| format!("{error:?}"))
+                            )))
+                        }
+                    }
+                }
+            });
         },
     );
     let _ = document.add_event_listener_with_callback("drop", on_drop.as_ref().unchecked_ref());
@@ -133,11 +172,3 @@ fn is_drawing_name(name: &str) -> bool {
         matches!(extension.to_ascii_lowercase().as_str(), "dwg" | "dxf")
     })
 }
-
-/// Same Blob → bytes read as the OPFS recent copies (`web_recent`).
-async fn read_file(file: &web_sys::File) -> Result<Vec<u8>, String> {
-    let buffer = JsFuture::from(file.array_buffer())
-        .await
-        .map_err(|error| error.as_string().unwrap_or_else(|| format!("{error:?}")))?;
-    Ok(js_sys::Uint8Array::new(&buffer).to_vec())
-}

src/io/web_recent.rs

@@ -90,10 +90,7 @@ pub async fn store_open(
                 return None;
             };
             if state.latest_id != pending_id {
-                return Some((
+                return Some((state.latest_id, std::sync::Arc::clone(&state.latest_bytes)));
-                    state.latest_id,
-                    std::sync::Arc::clone(&state.latest_bytes),
-                ));
             }
             state.active_writers = state.active_writers.saturating_sub(1);
             if state.active_writers == 0 {
@@ -130,6 +127,24 @@ pub(crate) async fn write_entry(
         .map_err(js_error)?
         .dyn_into::<web_sys::FileSystemWritableFileStream>()
         .map_err(js_error)?;
+    if let Err(error) = write_and_close(&writable, bytes).await {
+        // A failed write must still release the stream: the
+        // FileSystemWritableFileStream holds an exclusive lock on the entry,
+        // and leaving it undrained makes every later write to the same file
+        // fail the same way — one quota error would freeze autosave (its
+        // round would never finish) for the rest of the session. `abort`
+        // discards the pending bytes; ignore its own failure, there is
+        // nothing better to report than `error`.
+        let _ = JsFuture::from(writable.abort()).await;
+        return Err(error);
+    }
+    Ok(())
+}
+
+async fn write_and_close(
+    writable: &web_sys::FileSystemWritableFileStream,
+    bytes: &[u8],
+) -> Result<(), String> {
     let write = writable.write_with_u8_array(bytes).map_err(js_error)?;
     JsFuture::from(write).await.map_err(js_error)?;
     JsFuture::from(writable.close()).await.map_err(js_error)?;
@@ -262,9 +277,22 @@ fn decode_thumbnail(bytes: Vec<u8>) -> Option<Thumbnail> {
 
 /// Shared with the autosave snapshot store (`web_autosave`).
 pub(crate) fn js_error(value: wasm_bindgen::JsValue) -> String {
-    value
+    if let Some(text) = value.as_string() {
-        .as_string()
+        return text;
-        .unwrap_or_else(|| format!("browser storage error: {value:?}"))
+    }
+    // OPFS failures surface as DOMExceptions, which stringify as
+    // "[object DOMException]" — useless in a command-line warning. Their
+    // `name`/`message` properties carry the diagnosis ("QuotaExceededError",
+    // "NotFoundError", …), so render those when present.
+    let property = |key: &str| {
+        js_sys::Reflect::get(&value, &wasm_bindgen::JsValue::from_str(key))
+            .ok()
+            .and_then(|found| found.as_string())
+    };
+    match (property("name"), property("message")) {
+        (Some(name), Some(message)) => format!("{name}: {message}"),
+        _ => format!("browser storage error: {value:?}"),
+    }
 }
 
 /// Last-modified time (ms since the Unix epoch) of a drawing's cached copy,