refactor(ComparisonView): size crossfade window per line

src/widgets/ComparisonView/ui/Line/Line.svelte

@@ -10,6 +10,7 @@
 import {
     type ComparisonLine,
     computeLineRenderModel,
+    windowSizeForLine,
 } from '$entities/Font';
 import { getTypographySettingsStore } from '$features/AdjustTypography';
 import { getComparisonStore } from '../../model';
@@ -24,16 +25,14 @@ interface Props {
      * Count of chars the slider has passed, from `findSplitIndex`.
      */
     split: number;
-    /**
-     * Number of chars in the crossfade window around the split.
-     */
-    windowSize: number;
 }
 
-let { line, split, windowSize }: Props = $props();
+let { line, split }: Props = $props();
 
 const comparisonStore = getComparisonStore();
 
+const windowSize = $derived(windowSizeForLine(line.chars.length));
+
 const model = $derived(computeLineRenderModel(line, split, windowSize));
 
 const typography = getTypographySettingsStore();