feat(pdf): restore smooth crossfade for Chrome 111+ using document.startViewTransition

The original code used container.startViewTransition(setDocument) with a
synchronous callback, giving a 250ms CSS crossfade that looked smooth when
the PDF happened to re-render before the animation ended — but was a race.

Now there are three tiers:
- Chrome 126+: element-level startViewTransition, async, waits for pagerendered
- Chrome 111+ (Brave 138, Edge 111+): document-level startViewTransition with
  view-transition-name scoped to the PDF container, same async pattern
- Firefox / Safari / older Chromium: canvas snapshot overlay (no animation,
  but seamless — introduced in build #108)

The document-level path restores the smooth fade the user saw on Edge build
#93, now guaranteed to crossfade old→new rather than old→blank.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

services/web/frontend/js/features/pdf-preview/util/pdf-js-wrapper.ts

@@ -118,9 +118,25 @@ export default class PDFJSWrapper {
       })
 
       const container = this.container as typeof this.container & {
-        // supported since Chrome 126
+        // element-level View Transitions (Level 2), Chrome 126+
         startViewTransition?: (cb: () => void | Promise<void>) => {
           ready: Promise<void>
+          finished: Promise<void>
+        }
+      }
+      const doc_ = document as typeof document & {
+        // document-level View Transitions (Level 1), Chrome 111+
+        startViewTransition?: (cb: () => void | Promise<void>) => {
+          ready: Promise<void>
+          finished: Promise<void>
+        }
+      }
+
+      // Shared error handler for transition.ready rejections.
+      const onTransitionError = (err: any) => {
+        // InvalidStateError just means the document was hidden during the transition.
+        if (err?.name !== 'InvalidStateError') {
+          captureException(err, { tags: { handler: 'pdf-preview' } })
         }
       }
 
@@ -128,27 +144,33 @@ export default class PDFJSWrapper {
         typeof container.startViewTransition === 'function' &&
         document.visibilityState !== 'hidden'
       ) {
-        // Chrome 126+: element-level View Transition API.
-        // The async callback holds the "before" snapshot until pagerendered fires,
-        // so the crossfade goes old→new rather than old→blank.
+        // Chrome 126+: element-level View Transition, scoped to this container.
+        // Async callback holds the "before" snapshot until pagerendered fires.
         const transition = container.startViewTransition(async () => {
           setDocument()
           await firstPageRendered
         })
-
-        transition.ready.catch(err => {
-          // ignore InvalidStateError, it just means the document was hidden
-          if (err?.name !== 'InvalidStateError') {
-            captureException(err, {
-              tags: { handler: 'pdf-preview' },
-            })
-          }
+        transition.ready.catch(onTransitionError)
+      } else if (
+        typeof doc_.startViewTransition === 'function' &&
+        document.visibilityState !== 'hidden'
+      ) {
+        // Chrome 111+ / Edge 111+: document-level View Transition.
+        // Give the PDF container a unique view-transition-name so only it
+        // crossfades; the rest of the page stays static.
+        const name = 'ol-pdf-viewer'
+        this.container.style.viewTransitionName = name
+        const transition = doc_.startViewTransition(async () => {
+          setDocument()
+          await firstPageRendered
         })
+        transition.finished.finally(() => {
+          this.container.style.viewTransitionName = ''
+        })
+        transition.ready.catch(onTransitionError)
       } else {
-        // All other browsers: snapshot the currently-rendered canvases into a
-        // temporary overlay so setDocument()'s synchronous canvas-clear doesn't
-        // produce a white flash.  The overlay is removed once the first page of
-        // the new document has been painted.
+        // Firefox / Safari / very old Chromium: canvas snapshot overlay keeps
+        // the old content visible while setDocument() clears and repaints.
         const snap = this._snapshotCanvases()
         setDocument()
         if (snap) {