feat(modal): drag dialogs by a title-bar grip, clamped to the window

In-canvas modals were locked to screen-centre. Add a green move-arrow
grip beside the ✕ that drags the dialog; positioning is an offset from
centre applied via asymmetric padding. The drag offset is clamped to the
window so a dialog stops at the edge instead of being squeezed, and the
offset resets when the dialog closes so each opens centred.

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

src/app/mod.rs

@@ -285,6 +285,14 @@ pub(super) struct OpenCADStudio {
     /// The open in-canvas modal dialog, if any (Plan B: shared overlay instead
     /// of OS windows).
     active_modal: Option<ModalKind>,
+    /// Pixel offset of the active modal from screen-centre (drag-to-move).
+    /// Reset to zero whenever a modal closes so each dialog opens centred.
+    modal_offset: iced::Vector,
+    /// Cursor position from the previous drag-move while the modal title bar is
+    /// held; `None` before the first move of a drag.
+    modal_drag_last: Option<Point>,
+    /// True while the modal title bar is held (a drag is in progress).
+    modal_dragging: bool,
     /// New-release notification window — opened on startup when the
     /// GitHub releases API reports a newer version than this build.
     /// First-launch "make Open CAD Studio the default for .dwg/.dxf?" prompt
@@ -999,6 +1007,12 @@ pub enum Message {
     AboutOpen,
     /// Close whatever in-canvas modal dialog is open (Plan B).
     CloseModal,
+    /// Title-bar pressed: begin dragging the active modal.
+    ModalGrab,
+    /// Cursor moved while dragging the modal title bar.
+    ModalDragMove(Point),
+    /// Title-bar released: stop dragging.
+    ModalDragRelease,
     AboutCopyInfo,
     // ── Plugin Manager window ───────────────────────────────────────────
     PluginManagerOpen,
@@ -1411,6 +1425,9 @@ impl OpenCADStudio {
             main_window: None,
             color_pick_target: None,
             active_modal: None,
+            modal_offset: iced::Vector::ZERO,
+            modal_drag_last: None,
+            modal_dragging: false,
             default_assoc_prompted: false,
             update_notice_version: None,
             update_notice_body: None,

src/app/update.rs

@@ -89,6 +89,10 @@ impl OpenCADStudio {
             _ => {}
         }
         self.active_modal = None;
+        // Recentre the next dialog and drop any in-progress drag.
+        self.modal_offset = iced::Vector::ZERO;
+        self.modal_drag_last = None;
+        self.modal_dragging = false;
     }
 
     pub fn update(&mut self, msg: Message) -> Task<Message> {
@@ -5376,6 +5380,38 @@ impl OpenCADStudio {
                 self.close_active_modal();
                 Task::none()
             }
+            Message::ModalGrab => {
+                // Start a drag; the first ModalDragMove seeds the reference.
+                self.modal_dragging = true;
+                self.modal_drag_last = None;
+                Task::none()
+            }
+            Message::ModalDragMove(p) => {
+                if self.modal_dragging {
+                    if let Some(last) = self.modal_drag_last {
+                        self.modal_offset.x += p.x - last.x;
+                        self.modal_offset.y += p.y - last.y;
+                        // Clamp so the dialog stops at the window edge instead
+                        // of being squeezed (the off-centre padding shrinks the
+                        // dialog once it overlaps a border).
+                        if let Some((cw, ch)) = self.modal_outer_size() {
+                            let ww = self.vp_size.0 + 440.0;
+                            let wh = self.vp_size.1;
+                            let max_x = ((ww - cw) * 0.5).max(0.0);
+                            let max_y = ((wh - ch) * 0.5).max(0.0);
+                            self.modal_offset.x = self.modal_offset.x.clamp(-max_x, max_x);
+                            self.modal_offset.y = self.modal_offset.y.clamp(-max_y, max_y);
+                        }
+                    }
+                    self.modal_drag_last = Some(p);
+                }
+                Task::none()
+            }
+            Message::ModalDragRelease => {
+                self.modal_dragging = false;
+                self.modal_drag_last = None;
+                Task::none()
+            }
 
             Message::AboutCopyInfo => {
                 let info = format!(

src/app/view.rs

@@ -1050,7 +1050,9 @@ impl OpenCADStudio {
         // Former pop-up windows render as overlays here, so they work on both
         // the native (single main window) and web builds.
         let base: Element<'_, Message> = match self.modal_content() {
-            Some(content) => crate::ui::modal::modal(composed, content, Message::CloseModal),
+            Some(content) => {
+                crate::ui::modal::modal(composed, content, Message::CloseModal, self.modal_offset)
+            }
             None => composed.into(),
         };
         // The colour picker is a nested modal: it stacks over whichever dialog
@@ -1064,12 +1066,44 @@ impl OpenCADStudio {
                 .width(iced::Length::Fixed(420.0))
                 .height(iced::Length::Fixed(470.0)),
                 Message::CloseColorPicker,
+                iced::Vector::ZERO,
             )
         } else {
             base
         }
     }
 
+    /// Outer pixel size (content + title-bar/padding chrome) of the active
+    /// modal, used to clamp drag so it cannot be pushed off-screen. Mirrors the
+    /// `sized(..)` dimensions in [`Self::modal_content`]; keep the two in sync.
+    /// `None` has no active modal. About (content-sized) uses a safe estimate.
+    pub(crate) fn modal_outer_size(&self) -> Option<(f32, f32)> {
+        use super::ModalKind::*;
+        // Title bar (~26) + spacing (6) + frame padding (10·2) → ~52 vertical;
+        // frame padding → ~20 horizontal.
+        const EXTRA_W: f32 = 20.0;
+        const EXTRA_H: f32 = 52.0;
+        let (w, h) = match self.active_modal? {
+            About => (440, 360),
+            Shortcuts => (720, 520),
+            PluginManager => (520, 460),
+            UpdateNotice => (560, 460),
+            Layers => (900, 360),
+            PageSetup => (520, 460),
+            LayoutManager => (640, 320),
+            Plotstyle => (780, 540),
+            TextStyle => (860, 480),
+            MlStyle => (620, 420),
+            TableStyle => (620, 420),
+            MLeaderStyle => (560, 560),
+            DimStyle => (720, 560),
+            AssocPrompt => (440, 210),
+            Unsaved => (420, 160),
+            SaveDialog => (560, 480),
+        };
+        Some((w as f32 + EXTRA_W, h as f32 + EXTRA_H))
+    }
+
     /// Build the currently-open modal dialog's content (Plan B), or `None`.
     /// Each former pop-up window is constructed here and given a bounded size
     /// (About shrinks to its content). Rendered as an overlay by `view_main`.