feat(ui): anchor every ribbon dropdown and status-bar popup to its widget, flipping at window edges

All ribbon dropdowns (tool dropdowns, undo/redo history, and the layer /
colour / linetype / lineweight / style combos) and all status-bar popups
(scale, units, OSNAP, isolate, filter, customization menu, layout list)
now open directly at their widget instead of at a layout-estimated
position, and flip to the other side when the panel would cross a window
edge.

Each dropdown/pill widget is wrapped in a transparent PosReport that
records its screen bounds into a thread-local map on draw. The overlays
read those bounds:
- Ribbon: Ribbon::dd_anchor(id, panel_w, win_w) returns
  (align_right, h_pad, top); position_ribbon_dropdown places the panel
  below the widget, flipping right-aligned near the right edge.
- Status bar: popup builders take (pill_bounds, win_size) and call
  popup::position_statusbar_popup / popup_anchor, opening just above the
  pill and flipping left/right at the window edge.

The window size is now tracked (win_size, set on WindowResized). The old
layout estimators compute_dropdown_left / compute_layer_combo_left /
compute_prop_combo_left / compute_history_dropdown_left and
osnap_popup_right_offset are deleted.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/ui/popup/isolate_popup.rs

@@ -4,7 +4,7 @@
 //! in the viewport right-click menu.
 
 use iced::widget::{button, column, container, mouse_area, row, text};
-use iced::{Background, Border, Color, Element, Fill, Length, Padding, Theme};
+use iced::{Background, Border, Color, Element, Fill, Length, Rectangle, Theme};
 
 use crate::app::Message;
 
@@ -13,7 +13,12 @@ use crate::app::Message;
 ///
 /// - `has_selection`: enables Isolate / Hide (they act on the selection).
 /// - `isolation_active`: enables End Isolation (something is hidden).
-pub fn isolate_popup_overlay(has_selection: bool, isolation_active: bool) -> Element<'static, Message> {
+pub fn isolate_popup_overlay(
+    has_selection: bool,
+    isolation_active: bool,
+    pill: Option<Rectangle>,
+    win: (f32, f32),
+) -> Element<'static, Message> {
     let rows = column![
         action_row(
             "Isolate Objects",
@@ -44,17 +49,7 @@ pub fn isolate_popup_overlay(has_selection: bool, isolation_active: bool) -> Ele
         })
         .width(Length::Fixed(160.0));
 
-    let positioned = container(panel)
-        .align_right(Fill)
-        .align_bottom(Fill)
-        .padding(Padding {
-            bottom: 27.0,
-            right: 4.0,
-            top: 0.0,
-            left: 0.0,
-        })
-        .width(Fill)
-        .height(Fill);
+    let positioned = super::position_statusbar_popup(panel.into(), pill, win, 160.0, true);
 
     mouse_area(positioned)
         .on_press(Message::CloseIsolatePopup)

src/ui/popup/mod.rs

@@ -4,3 +4,76 @@ pub mod scale_popup;
 pub mod selection_filter_popup;
 pub mod snap_popup;
 pub mod units_popup;
+
+use iced::widget::container;
+use iced::{Element, Fill, Padding, Rectangle};
+
+use crate::app::Message;
+
+/// Decide a status-bar popup's horizontal anchor and paddings from its pill's
+/// screen bounds. Returns `(align_right, horizontal_pad, bottom_pad)`.
+///
+/// The popup opens just above the pill. Horizontally it prefers `prefer_right`
+/// (right edge aligned to the pill's right, growing left) but flips to the other
+/// side when that direction would cross the window edge.
+pub fn popup_anchor(
+    pill: Option<Rectangle>,
+    win: (f32, f32),
+    popup_w: f32,
+    prefer_right: bool,
+) -> (bool, f32, f32) {
+    let Some(b) = pill else {
+        // No recorded position yet — fall back to the bottom-right corner.
+        return (prefer_right, 4.0, 27.0);
+    };
+    let bottom = (win.1 - b.y).max(0.0);
+    let right_pad = (win.0 - (b.x + b.width)).max(4.0);
+    let left_pad = b.x.max(4.0);
+
+    if prefer_right {
+        // Right-aligned popup grows left; flip left-aligned if it would pass the
+        // left window edge.
+        if (b.x + b.width) - popup_w < 2.0 {
+            (false, left_pad, bottom)
+        } else {
+            (true, right_pad, bottom)
+        }
+    } else {
+        // Left-aligned popup grows right; flip right-aligned if it would pass the
+        // right window edge.
+        if b.x + popup_w > win.0 - 2.0 {
+            (true, right_pad, bottom)
+        } else {
+            (false, left_pad, bottom)
+        }
+    }
+}
+
+/// Wrap a status-bar popup `panel` in a full-window container positioned just
+/// above its pill, flipping horizontally to stay on screen.
+pub fn position_statusbar_popup<'a>(
+    panel: Element<'a, Message>,
+    pill: Option<Rectangle>,
+    win: (f32, f32),
+    popup_w: f32,
+    prefer_right: bool,
+) -> Element<'a, Message> {
+    let (align_right, h_pad, bottom) = popup_anchor(pill, win, popup_w, prefer_right);
+    let pad = Padding {
+        top: 0.0,
+        bottom,
+        left: if align_right { 0.0 } else { h_pad },
+        right: if align_right { h_pad } else { 0.0 },
+    };
+    let c = container(panel)
+        .align_bottom(Fill)
+        .width(Fill)
+        .height(Fill)
+        .padding(pad);
+    let c = if align_right {
+        c.align_right(Fill)
+    } else {
+        c.align_left(Fill)
+    };
+    c.into()
+}

src/ui/popup/scale_popup.rs

@@ -3,7 +3,7 @@
 //! Rendered as a floating overlay above the status bar, same pattern as snap_popup.
 
 use iced::widget::{button, column, container, mouse_area, row, text};
-use iced::{Background, Border, Color, Element, Fill, Length, Padding, Theme};
+use iced::{Background, Border, Color, Element, Fill, Length, Rectangle, Theme};
 
 use crate::app::Message;
 
@@ -20,6 +20,8 @@ pub fn scale_popup_overlay(
     current_anno_scale: f32,
     viewport_scale: Option<f64>,
     file_scales: Vec<(String, f32, f64)>,
+    pill: Option<Rectangle>,
+    win: (f32, f32),
 ) -> Element<'static, Message> {
     let rows: Vec<Element<'static, Message>> = file_scales
         .into_iter()
@@ -52,17 +54,7 @@ pub fn scale_popup_overlay(
         })
         .width(Length::Fixed(120.0));
 
-    let positioned = container(panel)
-        .align_right(Fill)
-        .align_bottom(Fill)
-        .padding(Padding {
-            bottom: 27.0,
-            right: 4.0,
-            top: 0.0,
-            left: 0.0,
-        })
-        .width(Fill)
-        .height(Fill);
+    let positioned = super::position_statusbar_popup(panel.into(), pill, win, 120.0, true);
 
     mouse_area(positioned)
         .on_press(Message::CloseScalePopup)

src/ui/popup/selection_filter_popup.rs

@@ -5,7 +5,7 @@
 use rustc_hash::FxHashSet as HashSet;
 
 use iced::widget::{button, column, container, mouse_area, row, text};
-use iced::{Background, Border, Color, Element, Fill, Length, Padding, Theme};
+use iced::{Background, Border, Color, Element, Fill, Length, Rectangle, Theme};
 
 use crate::app::Message;
 
@@ -17,6 +17,8 @@ use crate::app::Message;
 pub fn selection_filter_popup_overlay(
     types: Vec<String>,
     excluded: &HashSet<String>,
+    pill: Option<Rectangle>,
+    win: (f32, f32),
 ) -> Element<'static, Message> {
     // "Select All / Clear All" header, mirroring the OSNAP popup: Select All
     // clears every exclusion, Clear All excludes every present type.
@@ -70,17 +72,7 @@ pub fn selection_filter_popup_overlay(
         })
         .width(Length::Fixed(180.0));
 
-    let positioned = container(panel)
-        .align_right(Fill)
-        .align_bottom(Fill)
-        .padding(Padding {
-            bottom: 27.0,
-            right: 4.0,
-            top: 0.0,
-            left: 0.0,
-        })
-        .width(Fill)
-        .height(Fill);
+    let positioned = super::position_statusbar_popup(panel.into(), pill, win, 180.0, true);
 
     mouse_area(positioned)
         .on_press(Message::CloseSelectionFilterPopup)

src/ui/popup/snap_popup.rs

@@ -4,7 +4,7 @@
 //! inserted into the view stack when `snap_popup_open` is true.
 
 use iced::widget::{button, column, container, mouse_area, row, text};
-use iced::{Background, Border, Color, Element, Fill, Length, Padding, Theme};
+use iced::{Background, Border, Color, Element, Fill, Length, Rectangle, Theme};
 
 use crate::app::Message;
 use crate::snap::{SnapType, Snapper, ALL_SNAP_MODES};
@@ -12,7 +12,11 @@ use crate::snap::{SnapType, Snapper, ALL_SNAP_MODES};
 /// Returns a full-screen overlay element:
 ///   - a transparent click-catcher that closes the popup on outside click
 ///   - the popup panel pinned to the bottom-right (above the status bar)
-pub fn snap_popup_overlay<'a>(snapper: &'a Snapper, right_offset: f32) -> Element<'a, Message> {
+pub fn snap_popup_overlay<'a>(
+    snapper: &'a Snapper,
+    pill: Option<Rectangle>,
+    win: (f32, f32),
+) -> Element<'a, Message> {
     // ── Panel content ─────────────────────────────────────────────────────
     let all_on = snapper.all_on();
     let none_on = snapper.none_on();
@@ -55,23 +59,8 @@ pub fn snap_popup_overlay<'a>(snapper: &'a Snapper, right_offset: f32) -> Elemen
         })
         .width(Length::Fixed(PANEL_W));
 
-    // ── Position above the status bar, anchored by its bottom-LEFT corner at
-    // `right_offset` from the window's right edge (the Osnap button), so it
-    // opens up and to the right of the button. `align_right` positions by the
-    // panel's right edge, so pad by the anchor minus the panel width; clamp so
-    // a narrow bar can't push it off-screen. (#248)
-    let right = (right_offset - PANEL_W).max(4.0);
-    let positioned = container(panel)
-        .align_right(Fill)
-        .align_bottom(Fill)
-        .padding(Padding {
-            bottom: 27.0,
-            right,
-            top: 0.0,
-            left: 0.0,
-        })
-        .width(Fill)
-        .height(Fill);
+    // Anchored just above its pill, flipping horizontally to stay on screen.
+    let positioned = super::position_statusbar_popup(panel.into(), pill, win, PANEL_W, true);
 
     // ── Click-catcher: closes popup on any outside click ──────────────────
     mouse_area(positioned)

src/ui/popup/units_popup.rs

@@ -2,7 +2,7 @@
 //! floating overlay above the status bar, same pattern as the scale picker.
 
 use iced::widget::{button, column, container, mouse_area, row, text};
-use iced::{Background, Border, Color, Element, Fill, Length, Padding, Theme};
+use iced::{Background, Border, Color, Element, Fill, Length, Rectangle, Theme};
 
 use crate::app::Message;
 
@@ -37,7 +37,11 @@ pub fn unit_short(code: i16) -> &'static str {
 
 /// Full-screen overlay: transparent click-catcher + units list pinned
 /// bottom-right, above the status bar.
-pub fn units_popup_overlay(current: i16) -> Element<'static, Message> {
+pub fn units_popup_overlay(
+    current: i16,
+    pill: Option<Rectangle>,
+    win: (f32, f32),
+) -> Element<'static, Message> {
     let rows: Vec<Element<'static, Message>> = UNITS
         .iter()
         .map(|&(code, label)| unit_row(label, code == current, Message::SetDrawingUnits(code)))
@@ -55,17 +59,7 @@ pub fn units_popup_overlay(current: i16) -> Element<'static, Message> {
         })
         .width(Length::Fixed(140.0));
 
-    let positioned = container(panel)
-        .align_right(Fill)
-        .align_bottom(Fill)
-        .padding(Padding {
-            bottom: 27.0,
-            right: 4.0,
-            top: 0.0,
-            left: 0.0,
-        })
-        .width(Fill)
-        .height(Fill);
+    let positioned = super::position_statusbar_popup(panel.into(), pill, win, 140.0, true);
 
     mouse_area(positioned)
         .on_press(Message::CloseUnitsPopup)