fix: remeasure composer after hidden Core becomes visible

frontend/e2e/ui-visual-review.spec.ts

@@ -90,6 +90,50 @@ async function inspect(page: Page) {
   });
 }
 
+for (const route of ["/", "/?thoth_route=administration%2Fdatabase"]) {
+  test(`Core prompt remains readable after loading ${route}`, async ({ page }, testInfo) => {
+    const writes = await fixtures(page);
+    await page.goto(route);
+    if (route !== "/") {
+      await navigation(page);
+      await page.getByRole("button", { name: "Return to session", exact: true }).click();
+    }
+    const prompt = page.getByRole("textbox", { name: "New question", exact: true });
+    await expect(prompt).toBeVisible();
+    await prompt.focus();
+    const readable = async () => {
+      await expect.poll(() => prompt.evaluate(el => {
+        const style = getComputedStyle(el);
+        return el.clientHeight >= parseFloat(style.lineHeight) + parseFloat(style.paddingTop) + parseFloat(style.paddingBottom);
+      })).toBe(true);
+    };
+    await readable();
+    await expect(prompt).toHaveCSS("outline-style", "none");
+    await page.screenshot({ path: testInfo.outputPath("Prompt-focused.png") });
+    await prompt.fill("First line");
+    await prompt.press("Shift+Enter");
+    await prompt.press("a");
+    await expect(prompt).toHaveValue("First line\na");
+    await expect.poll(() => prompt.evaluate(el => el.clientHeight)).toBeGreaterThan(40);
+    const question = "Compare eligible patients across the available cohorts. ".repeat(30);
+    await prompt.fill(question);
+    await page.setViewportSize({ width: 390, height: 900 });
+    await readable();
+    await expect.poll(() => prompt.evaluate(el => el.clientHeight)).toBe(160);
+    expect(await prompt.evaluate(el => el.scrollHeight > el.clientHeight)).toBe(true);
+    await admin(page, "Database");
+    await navigation(page);
+    await page.getByRole("button", { name: "Return to session", exact: true }).click();
+    await expect(prompt).toHaveValue(question);
+    await readable();
+    await page.screenshot({ path: testInfo.outputPath("Prompt-multiline-mobile.png") });
+    await prompt.fill("");
+    await readable();
+    await expect.poll(() => prompt.evaluate(el => el.clientHeight)).toBeLessThanOrEqual(40);
+    expect(writes).toEqual([]);
+  });
+}
+
 for (const width of [390, 649, 768, 1280, 1600]) {
   test(`all surfaces retain shared typography at ${width}px`, async ({ page }, testInfo) => {
     await page.setViewportSize({ width, height: 1000 });

frontend/src/index.css

@@ -158,6 +158,11 @@
     outline-offset: 2px;
   }
 
+  /* The composer already exposes keyboard focus on its enclosing focus-within frame. */
+  #root textarea.thot-composer-input:focus-visible {
+    outline: none;
+  }
+
   #root :where(h1, h2, h3, h4, h5, h6),
   [data-slot="dialog-content"] :where(h1, h2, h3, h4, h5, h6) {
     font-family: var(--font-heading);

frontend/src/shell/SteerInput.tsx

@@ -69,12 +69,32 @@ export function SteerInput({
   }
 
   // Grow the textarea to fit its content (up to a cap, then it scrolls), and shrink
-  // back when the text is cleared. Runs after every value change so height tracks text.
+  // back when cleared. Core stays mounted while hidden behind context/Admin pages,
+  // so a hidden scrollHeight must never become a persisted zero-pixel height.
   useEffect(() => {
     const el = taRef.current;
     if (!el) return;
-    el.style.height = "auto";
-    el.style.height = `${Math.min(el.scrollHeight, 160)}px`;
+    let disposed = false;
+    const resize = () => {
+      if (disposed || el.getClientRects().length === 0) return;
+      el.style.height = "auto";
+      el.style.height = `${Math.min(el.scrollHeight, 160)}px`;
+    };
+    resize();
+    let lastWidth = -1;
+    const observer = typeof ResizeObserver === "undefined" ? undefined : new ResizeObserver(([entry]) => {
+      const width = entry.contentRect.width;
+      if (width === lastWidth) return;
+      lastWidth = width;
+      // Ignore height-only notifications from our own autosizing writes.
+      if (width > 0) resize();
+    });
+    observer?.observe(el);
+    void document.fonts?.ready.then(resize);
+    return () => {
+      disposed = true;
+      observer?.disconnect();
+    };
   }, [text]);
 
   async function submit() {
@@ -162,7 +182,7 @@ export function SteerInput({
         placeholder={sessionId ? "Free steering (!)…" : "Ask a question to start a new session…"}
         aria-label={sessionId ? "Steering" : "New question"}
         data-awaiting-input={awaitingInput ? "true" : undefined}
-        className={["max-h-40 flex-1 resize-none rounded-lg bg-card px-1 py-1 text-sm leading-relaxed outline-none placeholder:text-muted-foreground", awaitingInput && "thot-awaiting-input"].filter(Boolean).join(" ")}
+        className={["thot-composer-input min-h-8 min-w-0 max-h-40 flex-1 resize-none rounded-lg bg-card px-1 py-1 text-sm leading-relaxed outline-none placeholder:text-muted-foreground", awaitingInput && "thot-awaiting-input"].filter(Boolean).join(" ")}
       />
       <button
         type="button"