fix(web): preserve agent composer shrink sizing

web/src/components/agent/agent-conversation.tsx

@@ -46,6 +46,23 @@ function dayLabel(date: Date | null) {
   today.setDate(today.getDate() - 1);
   return dayKey(date) === dayKey(today) ? "Yesterday" : dayFormat.format(date);
 }
+function measureComposerHeight(textarea: HTMLTextAreaElement) {
+  const measurement = textarea.cloneNode(false) as HTMLTextAreaElement;
+  measurement.removeAttribute("id");
+  measurement.removeAttribute("name");
+  measurement.setAttribute("aria-hidden", "true");
+  measurement.tabIndex = -1;
+  measurement.value = textarea.value;
+  Object.assign(measurement.style, {
+    position: "fixed", left: "-10000px", top: "0", visibility: "hidden", pointerEvents: "none",
+    width: `${textarea.getBoundingClientRect().width}px`, height: "0px", minHeight: "0px",
+    maxHeight: "none", overflow: "hidden", resize: "none",
+  });
+  textarea.parentElement?.appendChild(measurement);
+  const height = measurement.scrollHeight;
+  measurement.remove();
+  return height;
+}
 function toolName(value: unknown) {
   return typeof value === "string" ? value.replace(/_/g, " ") : "Tool";
 }
@@ -288,12 +305,10 @@ export function AgentConversation({ room, onOpenSidebar, isVisible = true }: Pro
   useLayoutEffect(() => {
     const textarea = textareaRef.current;
     if (!textarea) return;
-    // scrollHeight reflects the full draft even when the textarea is already
-    // capped. Avoid collapsing a focused field to its minimum height while
-    // measuring: the browser may scroll the enclosing transcript to reveal its
-    // caret, and no ResizeObserver can correct that transient scroll once the
-    // field returns to the same capped height.
-    textarea.style.height = `${Math.min(156, Math.max(46, textarea.scrollHeight))}px`;
+    // Measure offscreen: collapsing a focused textarea can make its scroll
+    // parent reveal the caret, while its own scrollHeight cannot measure a
+    // smaller draft until its current height has been removed.
+    textarea.style.height = `${Math.min(156, Math.max(46, measureComposerHeight(textarea)))}px`;
   }, [draft]);
 
   const updateDraft = (value: string) => {

web/tests/agent-composer-layout.browser.mjs

@@ -107,11 +107,19 @@ try {
   if (process.env.RECORD_ONLY !== '1') for (const result of all) {
     for (const sample of result.bottom) assert.ok(Math.abs(sample.gap) <= 1,
       `bottom pin drift at ${result.config.width}px/${result.config.dpr}x: ${JSON.stringify(sample)}`);
+    const byLabel = Object.fromEntries(result.bottom.map(sample => [sample.label, sample]));
+    assert.equal(byLabel['wrapped-and-capped'].textareaHeight, 156, `textarea cap changed: ${JSON.stringify(result.bottom)}`);
+    assert.ok(byLabel.shrink.textareaHeight <= 48 && byLabel.cleared.textareaHeight <= 48,
+      `textarea did not shrink with the draft: ${JSON.stringify(result.bottom)}`);
+    assert.equal(byLabel.cleared.clientHeight, byLabel.empty.clientHeight,
+      `composer did not return to its empty height: ${JSON.stringify(result.bottom)}`);
     const before = result.nonbottom.find(sample => sample.before)?.before;
     for (const sample of result.nonbottom.filter(sample => sample.label?.startsWith('nonbottom-'))) {
       assert.equal(sample.anchorId, before.anchorId, `nonbottom anchor changed: ${JSON.stringify({ before, sample })}`);
       assert.ok(Math.abs(sample.anchorTop - before.anchorTop) <= 1, `nonbottom transcript jumped: ${JSON.stringify({ before, sample })}`);
     }
+    const nonbottomShrink = result.nonbottom.find(sample => sample.label === 'nonbottom-shrink');
+    assert.ok(nonbottomShrink.textareaHeight <= 48, `nonbottom textarea did not shrink: ${JSON.stringify(nonbottomShrink)}`);
   }
   process.stdout.write(JSON.stringify({ browser, all }, null, 2) + '\n');
 } finally {