feat(Font): add windowSizeForLine crossfade-window policy

src/entities/Font/domain/index.ts

@@ -8,3 +8,4 @@ export {
     findSplitIndex,
     type LineRenderModel,
 } from './computeLineRenderModel/computeLineRenderModel';
+export { windowSizeForLine } from './windowSizeForLine/windowSizeForLine';

src/entities/Font/domain/windowSizeForLine/windowSizeForLine.test.ts

@@ -0,0 +1,38 @@
+import {
+    describe,
+    expect,
+    it,
+} from 'vitest';
+import { windowSizeForLine } from './windowSizeForLine';
+
+describe('windowSizeForLine', () => {
+    it('returns 0 for an empty or non-positive line', () => {
+        expect(windowSizeForLine(0)).toBe(0);
+        expect(windowSizeForLine(-3)).toBe(0);
+    });
+
+    it('floors non-empty short lines at the minimum window of 1', () => {
+        expect(windowSizeForLine(1)).toBe(1);
+        expect(windowSizeForLine(2)).toBe(1);
+        expect(windowSizeForLine(3)).toBe(1);
+    });
+
+    it('scales with round(n / 3) in the mid range', () => {
+        expect(windowSizeForLine(6)).toBe(2);
+        expect(windowSizeForLine(12)).toBe(4);
+    });
+
+    it('caps at the maximum window of 5', () => {
+        expect(windowSizeForLine(15)).toBe(5);
+        expect(windowSizeForLine(16)).toBe(5);
+        expect(windowSizeForLine(100)).toBe(5);
+    });
+
+    it('rounds to nearest at fractional boundaries', () => {
+        // round(4/3)=1, round(5/3)=2, round(13/3)=4, round(14/3)=5
+        expect(windowSizeForLine(4)).toBe(1);
+        expect(windowSizeForLine(5)).toBe(2);
+        expect(windowSizeForLine(13)).toBe(4);
+        expect(windowSizeForLine(14)).toBe(5);
+    });
+});

src/entities/Font/domain/windowSizeForLine/windowSizeForLine.ts

@@ -0,0 +1,34 @@
+/**
+ * Crossfade-window sizing policy for the dual-font slider.
+ *
+ * The slider renders a band of per-char `Character` cells that opacity-crossfade
+ * between the two fonts; everything outside the band is committed native bulk
+ * text. A fixed band looked wrong on short lines — a 6-grapheme line left almost
+ * no bulk, so nearly the whole line shimmered as per-char DOM. The band size
+ * therefore scales with the line's grapheme count and caps so long lines don't
+ * pay for an oversized per-char DOM band.
+ */
+
+/**
+ * Fraction of a line's graphemes that sit in the crossfade band.
+ */
+const WINDOW_RATIO = 1 / 3;
+/**
+ * Smallest band for a non-empty line — guarantees at least one crossfading char.
+ */
+const WINDOW_MIN = 1;
+/**
+ * Largest band regardless of line length — bounds per-char DOM cost.
+ */
+const WINDOW_MAX = 5;
+
+/**
+ * Crossfade window size, in graphemes, for a line of `n` graphemes.
+ * `clamp(round(n / 3), 1, 5)`; an empty/non-positive line gets no window.
+ */
+export function windowSizeForLine(n: number): number {
+    if (n <= 0) {
+        return 0;
+    }
+    return Math.min(WINDOW_MAX, Math.max(WINDOW_MIN, Math.round(n * WINDOW_RATIO)));
+}

src/entities/Font/index.ts

@@ -2,6 +2,7 @@ export {
     computeLineRenderModel,
     DualFontLayout,
     findSplitIndex,
+    windowSizeForLine,
 } from './domain';
 export type {
     ComparisonLine,