fix: remeasure composer after hidden Core becomes visible

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"