Theme scrollbars + global broadcast toggle

Two small QoL additions:

- styles.css: WebKit pseudo-element styling for .xterm-viewport
  scrollbars (8px wide, dark thumb #2a2a2a on transparent track,
  hover lighten). Matches the rest of the dark theme so the right
  edge of each terminal stops looking like default OS chrome.

- tree.ts: setAllBroadcast(root, on) helper that flips every leaf's
  broadcast flag to the given value, preserving object identity
  where nothing changed.

- App.tsx: titlebar 📡 button showing global broadcast state
  ("all off" / "all on" / "N/M"). Click toggles every pane between
  all-broadcasting and all-off. Orange when any panes are
  broadcasting; darker orange when partial.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/App.css

@@ -47,6 +47,15 @@
   color: #cce6ff;
   border-color: #2a5a8c;
 }
+.palette-btn.bcast-all.on {
+  background: #4a3010;
+  color: #f0c060;
+  border-color: #c98a1f;
+}
+.palette-btn.bcast-all.on.partial {
+  background: #2a2010;
+  color: #c98a1f;
+}
 .preset-btn {
   min-width: 28px;
   text-align: center;

src/App.tsx

@@ -21,6 +21,7 @@ import {
   changeDistro,
   changeLabel,
   toggleBroadcast as toggleBroadcastInTree,
+  setAllBroadcast,
   serialize,
   deserialize,
   presetSingle,
@@ -282,6 +283,29 @@ export default function App() {
     [paletteOpen, tree],
   );
 
+  // ---- global broadcast state (derived from tree) -------------------------
+  const broadcastStats = useMemo(() => {
+    let on = 0;
+    let total = 0;
+    for (const leaf of walkLeaves(tree)) {
+      total++;
+      if (leaf.broadcast) on++;
+    }
+    return { on, total };
+  }, [tree]);
+
+  const toggleBroadcastAll = useCallback(() => {
+    // If any pane is broadcasting, turn them all off. Otherwise turn them all on.
+    setTree((t) => setAllBroadcast(t, broadcastStats.on === 0));
+  }, [broadcastStats.on]);
+
+  const broadcastBtnLabel =
+    broadcastStats.on === 0
+      ? "📡 all off"
+      : broadcastStats.on === broadcastStats.total
+        ? "📡 all on"
+        : `📡 ${broadcastStats.on}/${broadcastStats.total}`;
+
   const onPalettePick = useCallback((leafId: string) => {
     setActiveLeafId(leafId);
     setPaletteOpen(false);
@@ -321,6 +345,20 @@ export default function App() {
           <button className="preset-btn" title="2 × 2 grid" onClick={() => applyPreset(presetTwoByTwo)}>2×2</button>
         </span>
 
+        <button
+          className={`palette-btn bcast-all${broadcastStats.on > 0 ? " on" : ""}${broadcastStats.on > 0 && broadcastStats.on < broadcastStats.total ? " partial" : ""}`}
+          onClick={toggleBroadcastAll}
+          title={
+            broadcastStats.on === 0
+              ? "Click to broadcast input to ALL panes"
+              : broadcastStats.on === broadcastStats.total
+                ? "All panes broadcasting — click to disable"
+                : `${broadcastStats.on} of ${broadcastStats.total} panes broadcasting — click to disable all`
+          }
+        >
+          {broadcastBtnLabel}
+        </button>
+
         <button
           className="palette-btn"
           onClick={() => setPaletteOpen(true)}

src/lib/layout/tree.ts

@@ -198,6 +198,19 @@ export function toggleBroadcast(root: TreeNode, leafId: NodeId): TreeNode {
   });
 }
 
+/** Force every leaf's broadcast flag to `on`. Returns the same root reference
+ *  when nothing actually changed, so callers can skip a state update if so. */
+export function setAllBroadcast(root: TreeNode, on: boolean): TreeNode {
+  if (root.kind === "leaf") {
+    if (!!root.broadcast === on) return root;
+    return { ...root, broadcast: on };
+  }
+  const a = setAllBroadcast(root.a, on);
+  const b = setAllBroadcast(root.b, on);
+  if (a === root.a && b === root.b) return root;
+  return { ...root, a, b };
+}
+
 export function serialize(root: TreeNode): string {
   return JSON.stringify(root);
 }

src/styles.css

@@ -37,3 +37,29 @@ body {
 
 .xterm { height: 100%; }
 .xterm-viewport { background: #0c0c0c !important; }
+
+/* Themed scrollbars — Chromium pseudo-elements (WebView2 supports these). */
+.xterm-viewport::-webkit-scrollbar {
+  width: 8px;
+  height: 8px;
+}
+.xterm-viewport::-webkit-scrollbar-track {
+  background: transparent;
+}
+.xterm-viewport::-webkit-scrollbar-thumb {
+  background: #2a2a2a;
+  border-radius: 4px;
+  border: 1px solid #1a1a1a;
+}
+.xterm-viewport::-webkit-scrollbar-thumb:hover {
+  background: #3a3a3a;
+}
+.xterm-viewport::-webkit-scrollbar-corner {
+  background: transparent;
+}
+/* Firefox fallback (and the new spec) — not strictly needed in WebView2
+   but free-and-correct. */
+.xterm-viewport {
+  scrollbar-width: thin;
+  scrollbar-color: #2a2a2a transparent;
+}