feat(web): improve OCR overlay text fitting, reactivity, and accessibility

- Precise font sizing using canvas measureText instead of character-count heuristic
- Fix overlay repositioning on viewport resize by computing metrics from reactive state instead of DOM reads
- Fix animation delay on resize by using transition-colors instead of transition-all
- Add keyboard accessibility: OCR boxes are focusable via Tab with reading-order sort
- Show text on focus (same styling as hover) with proper ARIA attributes

web/src/lib/components/asset-viewer/ocr-bounding-box.svelte

@@ -1,6 +1,6 @@
 <script lang="ts">
   import type { OcrBox } from '$lib/utils/ocr-utils';
-  import { calculateBoundingBoxMatrix } from '$lib/utils/ocr-utils';
+  import { calculateBoundingBoxMatrix, calculateFittedFontSize } from '$lib/utils/ocr-utils';
 
   type Props = {
     ocrBox: OcrBox;
@@ -11,16 +11,16 @@
   const dimensions = $derived(calculateBoundingBoxMatrix(ocrBox.points));
 
   const transform = $derived(`matrix3d(${dimensions.matrix.join(',')})`);
-  // Fits almost all strings within the box, depends on font family
-  const fontSize = $derived(
-    `max(var(--text-sm), min(var(--text-6xl), ${(1.4 * dimensions.width) / ocrBox.text.length}px))`,
-  );
+  const fontSize = $derived(calculateFittedFontSize(ocrBox.text, dimensions.width, dimensions.height) + 'px');
 </script>
 
 <div class="absolute left-0 top-0">
   <div
-    class="absolute flex items-center justify-center text-transparent text-sm border-2 border-blue-500 bg-blue-500/10 px-2 py-1 pointer-events-auto cursor-text whitespace-pre-wrap wrap-break-word select-text transition-all hover:text-white hover:bg-black/60 hover:border-blue-600 hover:border-3"
+    class="absolute flex items-center justify-center text-transparent border-2 border-blue-500 bg-blue-500/10 px-2 py-1 pointer-events-auto cursor-text whitespace-nowrap select-text transition-colors hover:z-1 hover:text-white hover:bg-black/60 hover:border-blue-600 hover:border-3 focus:z-1 focus:text-white focus:bg-black/60 focus:border-blue-600 focus:border-3 focus:outline-none"
     style="font-size: {fontSize}; width: {dimensions.width}px; height: {dimensions.height}px; transform: {transform}; transform-origin: 0 0;"
+    tabindex="0"
+    role="button"
+    aria-label={ocrBox.text}
   >
     {ocrBox.text}
   </div>

web/src/lib/components/asset-viewer/photo-viewer.svelte

@@ -15,7 +15,7 @@
   import { SlideshowLook, SlideshowState, slideshowLookCssMapping, slideshowStore } from '$lib/stores/slideshow.store';
   import { getAssetUrl, targetImageSize as getTargetImageSize, handlePromiseError } from '$lib/utils';
   import { canCopyImageToClipboard, copyImageToClipboard } from '$lib/utils/asset-utils';
-  import { type ContentMetrics, getContentMetrics } from '$lib/utils/container-utils';
+  import { type ContentMetrics, getNaturalSize, scaleToFit } from '$lib/utils/container-utils';
   import { handleError } from '$lib/utils/handle-error';
   import { getOcrBoundingBoxes } from '$lib/utils/ocr-utils';
   import { getBoundingBox } from '$lib/utils/people-utils';
@@ -74,12 +74,16 @@
       return { contentWidth: 0, contentHeight: 0, offsetX: 0, offsetY: 0 };
     }
 
-    const { contentWidth, contentHeight, offsetX, offsetY } = getContentMetrics(assetViewerManager.imgRef);
+    const natural = getNaturalSize(assetViewerManager.imgRef);
+    const client = { width: containerWidth, height: containerHeight };
+    const scaled = scaleToFit(natural, client);
+    const offsetX = (client.width - scaled.width) / 2;
+    const offsetY = (client.height - scaled.height) / 2;
     const { currentZoom, currentPositionX, currentPositionY } = assetViewerManager.zoomState;
 
     return {
-      contentWidth: contentWidth * currentZoom,
-      contentHeight: contentHeight * currentZoom,
+      contentWidth: scaled.width * currentZoom,
+      contentHeight: scaled.height * currentZoom,
       offsetX: offsetX * currentZoom + currentPositionX,
       offsetY: offsetY * currentZoom + currentPositionY,
     };

web/src/lib/utils/ocr-utils.ts

@@ -1,5 +1,6 @@
 import type { OcrBoundingBox } from '$lib/stores/ocr.svelte';
 import type { ContentMetrics } from '$lib/utils/container-utils';
+import { clamp } from 'lodash-es';
 
 export type Point = {
   x: number;
@@ -55,6 +56,54 @@ export const calculateBoundingBoxMatrix = (points: Point[]): { matrix: number[];
   return { matrix, width, height };
 };
 
+const HORIZONTAL_PADDING = 16;
+const VERTICAL_PADDING = 8;
+const REFERENCE_FONT_SIZE = 100;
+const MIN_FONT_SIZE = 8;
+const MAX_FONT_SIZE = 96;
+const REFERENCE_FONT = `${REFERENCE_FONT_SIZE}px 'GoogleSans', sans-serif`;
+
+let sharedCanvasContext: CanvasRenderingContext2D | null = null;
+
+const getCanvasContext = (): CanvasRenderingContext2D | null => {
+  if (sharedCanvasContext !== null) {
+    return sharedCanvasContext;
+  }
+  if (typeof document === 'undefined') {
+    return null;
+  }
+  const canvas = document.createElement('canvas');
+  const context = canvas.getContext('2d');
+  if (!context) {
+    return null;
+  }
+  sharedCanvasContext = context;
+  return sharedCanvasContext;
+};
+
+export const calculateFittedFontSize = (text: string, boxWidth: number, boxHeight: number): number => {
+  const availableWidth = boxWidth - HORIZONTAL_PADDING;
+  const availableHeight = boxHeight - VERTICAL_PADDING;
+
+  const context = getCanvasContext();
+  if (!context) {
+    return clamp((1.4 * availableWidth) / text.length, MIN_FONT_SIZE, MAX_FONT_SIZE);
+  }
+
+  // Unsupported in Safari iOS <16.6; falls back to default canvas font, giving less accurate but functional sizing
+  // eslint-disable-next-line tscompat/tscompat
+  context.font = REFERENCE_FONT;
+
+  const metrics = context.measureText(text);
+  const measuredWidth = metrics.width;
+  const measuredHeight = metrics.actualBoundingBoxAscent + metrics.actualBoundingBoxDescent;
+
+  const scaleFromWidth = (availableWidth / measuredWidth) * REFERENCE_FONT_SIZE;
+  const scaleFromHeight = (availableHeight / measuredHeight) * REFERENCE_FONT_SIZE;
+
+  return clamp(Math.min(scaleFromWidth, scaleFromHeight), MIN_FONT_SIZE, MAX_FONT_SIZE);
+};
+
 export const getOcrBoundingBoxes = (ocrData: OcrBoundingBox[], metrics: ContentMetrics): OcrBox[] => {
   const boxes: OcrBox[] = [];
   for (const ocr of ocrData) {
@@ -76,5 +125,14 @@ export const getOcrBoundingBoxes = (ocrData: OcrBoundingBox[], metrics: ContentM
     });
   }
 
+  const rowThreshold = metrics.contentHeight * 0.02;
+  boxes.sort((a, b) => {
+    const yDifference = a.points[0].y - b.points[0].y;
+    if (Math.abs(yDifference) < rowThreshold) {
+      return a.points[0].x - b.points[0].x;
+    }
+    return yDifference;
+  });
+
   return boxes;
 };