fix: restore the jump-to-bottom glide and keep user state, selections and scroll anchors in windowed lists

Pause scroll writes while the jump-to-bottom smooth scroll runs; keep rows with pending questions, approvals, recent interactions and selection ends mounted; cache Mermaid results; copy text spanning unmounted rows; anchor the sidebar on id changes; disable native overflow-anchor for windowed lists; write terminal output on the leading edge and before clear; forward remote events every 16 ms; flush streaming text before copy/export; add performance principle P10 (no UX regressions).

tests/support/app-perf-driver.mjs

@@ -607,7 +607,26 @@ try {
     }
     await sleep(300)
     const after = await js(scroller)
-    return { fields: { scrollTopBefore: before, scrollTopAfter: after, scrolledPx: before === null || after === null ? null : before - after } }
+    // UX: large jumps (scrollbar drag, Home/End, flick) must not paint a frame
+    // whose viewport is not covered by mounted rows (a blank screen).
+    const blankFrames = await js(`new Promise((resolve) => {
+      const c = document.querySelector('.chat-history-pane[data-active=true] .chat');
+      const covered = () => {
+        const v = c.getBoundingClientRect(), mid = v.top + v.height / 2;
+        return [...c.querySelectorAll('.message-window-row')].some((row) => { const r = row.getBoundingClientRect(); return r.top <= mid && r.bottom >= mid; });
+      };
+      const targets = [0.5, 0.1, 0.9, 0.3, 0.7, 0.05, 0.95, 0.2, 0.8, 0.4].map((f) => Math.round((c.scrollHeight - c.clientHeight) * f));
+      let blank = 0, checked = 0;
+      const step = () => {
+        const target = targets.shift();
+        if (target === undefined) { resolve({ blank, checked }); return; }
+        c.scrollTop = target;
+        // The frame the jump is painted in: rAF runs before paint, after rendering work queued by the scroll.
+        requestAnimationFrame(() => requestAnimationFrame(() => { checked += 1; if (!covered()) blank += 1; setTimeout(step, 50); }));
+      };
+      step();
+    })`)
+    return { fields: { scrollTopBefore: before, scrollTopAfter: after, scrolledPx: before === null || after === null ? null : before - after, jumpBlankFrames: blankFrames.blank, jumpChecks: blankFrames.checked } }
   }))
 
   await screenshot('long-conversation')

tests/support/frosted-glass-driver.mjs

@@ -163,8 +163,22 @@ app.whenReady().then(async () => {
       const clicks = await evaluate(`Number(document.querySelector('[data-clicks]').dataset.clicks)`)
       await click('.topbar button')
       await waitFor(`Number(document.querySelector('[data-clicks]').dataset.clicks) === ${clicks + 1}`)
+      // The jump-to-bottom button glides (UX): sample scrollTop each frame after
+      // the click; an instant jump would show no position between start and end.
+      await evaluate(`(() => { const e = document.querySelector('.chat-history-pane[data-active="true"] .chat');
+        const samples = window.__jumpSamples = [e.scrollTop];
+        const sample = () => { samples.push(e.scrollTop); if (samples.length < 90) requestAnimationFrame(sample); };
+        requestAnimationFrame(sample); })()`)
       await click('.chat-scroll-to-bottom')
       await waitFor(`(() => { const e = document.querySelector('.chat-history-pane[data-active="true"] .chat'); return e.scrollHeight - e.clientHeight - e.scrollTop < 1; })()`)
+      await waitFor('window.__jumpSamples.length >= 90')
+      const glide = await evaluate(`(() => { const e = document.querySelector('.chat-history-pane[data-active="true"] .chat');
+        const end = e.scrollHeight - e.clientHeight, s = window.__jumpSamples, start = s[0];
+        const between = new Set(s.filter(v => v > start + 1 && v < end - 1).map(Math.round));
+        const reappeared = !!document.querySelector('.chat-scroll-to-bottom');
+        return { start, end, between: between.size, reappeared, samples: s.slice(0, 12).map(Math.round) }; })()`)
+      assert.ok(glide.between >= 3, `${theme}/${enabled}: jump-to-bottom animates ${JSON.stringify(glide)}`)
+      assert.equal(glide.reappeared, false, `${theme}/${enabled}: button stays hidden after the glide`)
       const bottom = await evaluate(`document.querySelector('.chat-history-pane[data-active="true"] article:last-child').getBoundingClientRect().bottom`)
       assert.ok(bottom <= geometry.composer.top, 'last message is above composer')
       assert.equal(await evaluate(`document.querySelector('[aria-label="Composer"]').value`), 'Draft')