fix(pdf): replace document.startViewTransition with non-blocking canvas fade

document.startViewTransition with an async callback places a ::view-transition
overlay on top of the entire page, intercepting pointer events for the duration
of the callback (up to the 1s safety timeout + 250ms animation).  With rapid
auto-compiles this created interface freezes and overlapping transitions that
could leave the visual lock in a broken state, causing 'stuck on compiling'.

Replace with a canvas snapshot overlay + CSS opacity fade-out:
- pointer-events:none so the overlay never blocks input
- snapshot covers the canvas-clear from setDocument() (no white flash)
- on pagerendered: opacity transitions to 0 over 250ms, then overlay removed
- gives the same smooth visual crossfade, reliably, in all browsers

Chrome 126+ retains the element-level startViewTransition path which is
scoped to the PDF container and does not affect the rest of the page.

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

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

@@ -121,22 +121,6 @@ export default class PDFJSWrapper {
         // 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' } })
         }
       }
 
@@ -145,44 +129,31 @@ export default class PDFJSWrapper {
         document.visibilityState !== 'hidden'
       ) {
         // Chrome 126+: element-level View Transition, scoped to this container.
-        // Async callback holds the "before" snapshot until pagerendered fires.
+        // The async callback holds the captured "before" state until the first
+        // new page is rendered, so the crossfade goes old→new, not old→blank.
         const transition = container.startViewTransition(async () => {
           setDocument()
           await firstPageRendered
         })
-        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.  We also suppress the auto-generated root-level animation
-        // (which would otherwise fade the entire page including the editor) by
-        // injecting a temporary <style> that sets animation:none on the root
-        // pseudo-elements.  The style is removed after the transition finishes.
-        const name = 'ol-pdf-viewer'
-        this.container.style.viewTransitionName = name
-        const noRootAnim = document.createElement('style')
-        noRootAnim.textContent =
-          '::view-transition-old(root),::view-transition-new(root){animation:none}'
-        document.head.appendChild(noRootAnim)
-        const transition = doc_.startViewTransition(async () => {
-          setDocument()
-          await firstPageRendered
+        transition.ready.catch(err => {
+          if (err?.name !== 'InvalidStateError') {
+            captureException(err, { tags: { handler: 'pdf-preview' } })
+          }
         })
-        transition.finished.finally(() => {
-          this.container.style.viewTransitionName = ''
-          noRootAnim.remove()
-        })
-        transition.ready.catch(onTransitionError)
       } else {
-        // Firefox / Safari / very old Chromium: canvas snapshot overlay keeps
-        // the old content visible while setDocument() clears and repaints.
+        // All other browsers: canvas snapshot overlay.
+        // The snapshot covers the canvas-clear that setDocument() triggers so
+        // the viewer never flashes white.  Once the first new page is rendered
+        // the overlay fades out (CSS opacity transition), giving the same smooth
+        // visual crossfade without ever blocking user input (pointer-events:none).
         const snap = this._snapshotCanvases()
         setDocument()
         if (snap) {
-          firstPageRendered.then(() => snap.remove())
+          firstPageRendered.then(() => {
+            snap.style.transition = 'opacity 0.25s ease'
+            snap.style.opacity = '0'
+            setTimeout(() => snap.remove(), 280)
+          })
         }
       }