quick fix to compare (#4893)

# Description of Changes

- Compare fix to stop the pages from getting stuck in loading state. 

This code tracks which page images have actually finished loading in a
Map stored in a ref, and then uses a throwaway useState value
(setImageLoadedTick) just to force a re-render when that ref changes.
When an <img> fires onLoad, it marks that page as loaded in
imageLoadedRef and bumps the tick, causing React to re-render and
re-evaluate the condition for the loader overlay. Because the overlay is
only shown when imageLoadedRef.current.get(page.pageNumber) is false,
that re-render hides the blur/loader once the page’s image has loaded
and is in view, instead of the UI getting stuck in the “loading” state.

<img width="823" height="984" alt="Screenshot 2025-11-13 at 5 57 42 PM"
src="https://github.com/user-attachments/assets/93445208-d653-4d89-b33a-aebea69c97d9"
/>

---

## Checklist

### General

- [ ] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [ ] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md)
(if applicable)
- [ ] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md)
(if applicable)
- [ ] I have performed a self-review of my own code
- [ ] My changes generate no new warnings

### Documentation

- [ ] I have updated relevant docs on [Stirling-PDF's doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
(if functionality has heavily changed)
- [ ] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### Translations (if applicable)

- [ ] I ran
[`scripts/counter_translation.py`](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/docs/counter_translation.md)

### UI Changes (if applicable)

- [ ] Screenshots or videos demonstrating the UI changes are attached
(e.g., as comments or direct attachments in the PR)

### Testing (if applicable)

- [ ] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md#6-testing)
for more details.

frontend/src/core/components/tools/compare/CompareDocumentPane.tsx

@@ -1,5 +1,5 @@
 import { Group, Loader, Stack, Text } from '@mantine/core';
-import { useMemo, useRef, useEffect } from 'react';
+import { useMemo, useRef, useEffect, useState } from 'react';
 import type { PagePreview } from '@app/types/compare';
 import type { TokenBoundingBox, CompareDocumentPaneProps } from '@app/types/compare';
 import { mergeConnectedRects, normalizeRotation, groupWordRects, computePageLayoutMetrics } from '@app/components/tools/compare/compare';
@@ -53,6 +53,8 @@ const CompareDocumentPane = ({
 
   // Track which page images have finished loading to avoid flashing between states
   const imageLoadedRef = useRef<Map<number, boolean>>(new Map());
+  // Force a re-render when an image load state changes (refs don't trigger renders)
+  const [, setImageLoadedTick] = useState(0);
   const visiblePageRafRef = useRef<number | null>(null);
   const lastReportedVisiblePageRef = useRef<number | null>(null);
   const pageNodesRef = useRef<HTMLElement[] | null>(null);
@@ -252,6 +254,7 @@ const CompareDocumentPane = ({
                         onLoad={() => {
                           if (!imageLoadedRef.current.get(page.pageNumber)) {
                             imageLoadedRef.current.set(page.pageNumber, true);
+                            setImageLoadedTick((v) => v + 1); // refs don't trigger renders
                           }
                         }}
                       />