fix(chrome): TitleBar drag via explicit window_start_drag (official recipe)

data-tauri-drag-region only fires when the exact element is the event target
and was never runtime-verified; replace it with the official Tauri custom-
titlebar recipe — primary-button mousedown starts an OS drag, detail===2
toggles maximize. Works across the whole region (brand text included, which
already passes pointer events through).

Pairs with cinny-desktop set_custom_chrome Mica fix (clear backdrop before
undecorating; window-state no longer restores the decorated flag).

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

src/app/features/desktop/TitleBar.tsx

@@ -66,8 +66,9 @@ function ControlButton({ label, glyph, onClick, close }: ControlButtonProps) {
  *
  * Renders `null` unless we're inside Tauri **and** the user opted into custom
  * window chrome. Otherwise it draws a thin (~32px) folds/TDS-styled titlebar: a
- * draggable region (`data-tauri-drag-region`) with the app brand, plus
- * minimize / maximize / close controls that call the native window commands.
+ * draggable region (explicit `window_start_drag` on mousedown, double-press to
+ * maximize) with the app brand, plus minimize / maximize / close controls that
+ * call the native window commands.
  *
  * OS-aware: Windows/Linux put the controls on the right; macOS mirrors them to
  * the left (the native traffic-light position) since decorations — and thus the
@@ -80,10 +81,18 @@ export function TitleBar() {
 
   const mac = isMacOS();
 
-  const handleDoubleClick = (evt: MouseEvent<HTMLDivElement>): void => {
-    // Only the drag surface itself toggles maximize, not the brand/children.
-    if (evt.target !== evt.currentTarget) return;
+  // Official Tauri custom-titlebar recipe: primary-button mousedown starts an
+  // OS window drag; a double press (detail === 2) toggles maximize instead. An
+  // explicit `window_start_drag` invoke is used rather than
+  // `data-tauri-drag-region` because the attribute only fires when the exact
+  // element is the event target (children like the brand text wouldn't drag).
+  const handleDragMouseDown = (evt: MouseEvent<HTMLDivElement>): void => {
+    if (evt.button !== 0) return;
+    if (evt.detail === 2) {
       invokeTauri('window_toggle_maximize');
+    } else {
+      invokeTauri('window_start_drag');
+    }
   };
 
   const controls = (
@@ -108,7 +117,7 @@ export function TitleBar() {
   );
 
   const dragRegion = (
-    <div className={css.DragRegion} data-tauri-drag-region onDoubleClick={handleDoubleClick}>
+    <div className={css.DragRegion} onMouseDown={handleDragMouseDown}>
       <span className={css.Brand}>
         <Text as="span" size="T200" truncate>
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