Inspector: Fix Firefox error reading child window bounds after close (#18389)

> 🤖 *This PR was created by the create-pr skill.*

## Problem

In `ChildWindow`
(`packages/dev/sharedUiComponents/src/fluent/hoc/childWindow.tsx`), the
dispose action that persists the child window's bounds to `localStorage`
reads `screenX`/`screenY`/`innerWidth`/`innerHeight` directly off the
child `Window`. In Firefox, accessing these properties after the window
has closed throws — which is exactly what happens when the user closes
the popout (the `unload` handler clears `childWindow` state, the effect
re-runs, and the cleanup fires while `childWindow.closed === true`).

## Fix

Cache the bounds while the window is still alive and use the cached
value at dispose time:

- Added a `getBounds()` helper.
- Initialize `lastBounds` eagerly when the window opens, so a value is
always available even if no later capture happens.
- Refresh `lastBounds` in a `beforeunload` listener (fires before the
window is destroyed in every close path).
- At dispose, refresh once more if the window is still open, then write
`lastBounds` to `localStorage`.

This eliminates the throw in Firefox while preserving the existing
bounds-restore behavior in all browsers.

## Files changed

- `packages/dev/sharedUiComponents/src/fluent/hoc/childWindow.tsx`

Author: ryantrem

packages/dev/sharedUiComponents/src/fluent/hoc/childWindow.tsx

@@ -191,6 +191,17 @@ export const ChildWindow: FunctionComponent<PropsWithChildren<ChildWindowProps>>
             setWindowState({ mountNode: body, renderer: createDOMRenderer(childWindow.document) });
             onOpenChange?.(true);
 
+            // Track the most recently observed window bounds. In some browsers (e.g. Firefox), accessing
+            // properties like screenX on a closed window throws, so we cache the last known good values
+            // to use as a fallback when the dispose runs after the window has already been closed.
+            const getBounds = () => ({
+                left: childWindow.screenX,
+                top: childWindow.screenY,
+                width: childWindow.innerWidth,
+                height: childWindow.innerHeight,
+            });
+            let lastBounds = getBounds();
+
             // When the child window is closed for any reason, transition back to a closed state.
             const onChildWindowUnload = () => {
                 setWindowState(undefined);
@@ -200,6 +211,13 @@ export const ChildWindow: FunctionComponent<PropsWithChildren<ChildWindowProps>>
             childWindow.addEventListener("unload", onChildWindowUnload, { once: true });
             disposeActions.push(() => childWindow.removeEventListener("unload", onChildWindowUnload));
 
+            // Capture bounds before the window is unloaded, while its properties are still safe to read.
+            const onChildWindowBeforeUnload = () => {
+                lastBounds = getBounds();
+            };
+            childWindow.addEventListener("beforeunload", onChildWindowBeforeUnload);
+            disposeActions.push(() => childWindow.removeEventListener("beforeunload", onChildWindowBeforeUnload));
+
             // If the main window closes, close any open child windows as well (don't leave them orphaned).
             const onParentWindowUnload = () => {
                 childWindow.close();
@@ -213,15 +231,10 @@ export const ChildWindow: FunctionComponent<PropsWithChildren<ChildWindowProps>>
             // On dispose, save the window bounds.
             disposeActions.push(() => {
                 if (storageKey) {
-                    localStorage.setItem(
-                        storageKey,
-                        JSON.stringify({
-                            left: childWindow.screenX,
-                            top: childWindow.screenY,
-                            width: childWindow.innerWidth,
-                            height: childWindow.innerHeight,
-                        })
-                    );
+                    if (!childWindow.closed) {
+                        lastBounds = getBounds();
+                    }
+                    localStorage.setItem(storageKey, JSON.stringify(lastBounds));
                 }
             });
         }