test(desktop): deselect-proof the code-scroll drag

The selection-drag assertion raced selection anchoring: mousedown on a
guessed container offset sometimes never formed a selection anchor
(cold-start font reflow moves the glyphs), so the long drag
auto-scrolled without selecting and failed with an opaque zero at the
end. Anchor on the measured first line after fonts settle, establish
the selection with a short in-viewport drag, and only start the long
auto-scroll drag once the selection observably exists.

Author: orangeCatDeveloper

apps/desktop/e2e/code-scroll.spec.ts

@@ -100,12 +100,25 @@ test('a one-line Markdown code block exposes native and selection horizontal scr
     (element as HTMLElement).scrollLeft = 0;
     window.getSelection()?.removeAllRanges();
   });
-  const code = viewport.locator('code');
-  const codeBox = await code.boundingBox();
-  if (!codeBox) throw new Error('code line has no visible bounds');
-  const textY = codeBox.y + Math.min(codeBox.height / 2, 18);
-  await page.mouse.move(codeBox.x + 24, textY);
+  // Anchor the drag on the first rendered character, not a guessed container
+  // offset: if mousedown lands in padding or between glyphs the selection
+  // anchor never forms, and the long drag then auto-scrolls without selecting
+  // (the historical flake in this test).
+  const line = viewport.locator('code > [data-line]').first();
+  // Cold-start guard: font loading reflows the glyphs, so a box measured
+  // before fonts settle puts the anchor on the wrong pixel.
+  await page.evaluate(() => document.fonts.ready.then(() => undefined));
+  const lineBox = await line.boundingBox();
+  if (!lineBox) throw new Error('code line has no visible bounds');
+  const textY = lineBox.y + Math.min(lineBox.height / 2, 18);
+  await page.mouse.move(lineBox.x + 24, textY);
   await page.mouse.down();
+  // Establish the selection with a short in-viewport drag before the long
+  // auto-scroll drag, and only proceed once the selection observably exists.
+  await page.mouse.move(lineBox.x + 90, textY, { steps: 6 });
+  await expect.poll(
+    () => viewport.evaluate(() => window.getSelection()?.toString().length ?? 0),
+  ).toBeGreaterThan(0);
   await page.mouse.move(metrics.rect.x + metrics.rect.width + 50, textY, { steps: 20 });
   await expect.poll(
     () => viewport.evaluate((element) => (element as HTMLElement).scrollLeft),