Per-pane and global terminal zoom via keyboard

Each leaf now carries an optional fontSizeOffset, persisted in
workspace.json alongside everything else. Ctrl+= / Ctrl+- / Ctrl+0
adjust the active pane; adding Shift escalates to every pane (the
mirror of the broadcast Shift+Alt convention, with shift alone since
the keys are otherwise unused). Bindings match on e.code so layouts
that don't have "=" / "-" / "0" in the same spot still work.

XtermPane gained a fontSize prop. A secondary effect reacts to changes:
set term.options.fontSize, fit() to recompute cols/rows for the new
cell size, refresh(), then resizePane so bash redraws the prompt at
the right width. No remount, so PTY + scrollback survive zoom changes.

The new tree helpers (resolveFontSize / adjustFontSize /
adjustAllFontSizes) are metadata-only — they don't swap leaf ids, so
nothing respawns. reshapeToPreset also carries the offset across when
splicing existing leaves into a new layout. 12 new vitest cases pin
those invariants plus the clamp and reset-to-default behaviour.

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

src/components/XtermPane.tsx

@@ -52,8 +52,12 @@ interface XtermPaneProps {
   onFocus?: () => void;
   /** Increment to refocus the terminal programmatically (palette etc.). */
   focusTrigger?: number;
+  /** Absolute font size in px. Changes are applied live (fit + PTY resize). */
+  fontSize?: number;
 }
 
+const DEFAULT_XTERM_FONT_SIZE = 13;
+
 // ---------------------------------------------------------------------------
 // Component
 // ---------------------------------------------------------------------------
@@ -67,8 +71,16 @@ export default function XtermPane({
   onDataReceived,
   onFocus,
   focusTrigger = 0,
+  fontSize,
 }: XtermPaneProps) {
   const containerRef = useRef<HTMLDivElement>(null);
+  const termRef = useRef<Terminal | null>(null);
+  const fitRef = useRef<FitAddon | null>(null);
+  const paneIdRef = useRef<PaneId | null>(null);
+  // Stash the most recent `fontSize` prop so the mount effect can pick
+  // up the initial value without re-running when it changes (the secondary
+  // effect below handles dynamic updates).
+  const initialFontSizeRef = useRef(fontSize);
 
   // Stable refs for callbacks so the mount effect doesn't need to re-run when
   // parents pass new inline functions, while still always calling the latest version.
@@ -93,7 +105,7 @@ export default function XtermPane({
 
     let term: Terminal | null = new Terminal({
       fontFamily: '"Cascadia Mono", "JetBrains Mono", "Consolas", monospace',
-      fontSize: 13,
+      fontSize: initialFontSizeRef.current ?? DEFAULT_XTERM_FONT_SIZE,
       cursorBlink: true,
       theme: {
         background: "#0c0c0c",
@@ -103,8 +115,10 @@ export default function XtermPane({
       convertEol: false,
       allowProposedApi: true,
     });
+    termRef.current = term;
 
     const fit = new FitAddon();
+    fitRef.current = fit;
     term.loadAddon(fit);
     term.open(container);
 
@@ -127,6 +141,7 @@ export default function XtermPane({
           void killPane(paneId);
           return;
         }
+        paneIdRef.current = paneId;
         onStatusRef.current?.(`pane ${paneId} alive`, true);
         onSpawnRef.current?.(paneId);
       } catch (e) {
@@ -219,6 +234,9 @@ export default function XtermPane({
       if (paneId != null) void killPane(paneId);
       term?.dispose();
       term = null;
+      termRef.current = null;
+      fitRef.current = null;
+      paneIdRef.current = null;
     };
     // distro/cwd are only used at spawn time; intentionally omitted from deps
     // so remounting doesn't happen if a parent re-renders with the same values.
@@ -228,13 +246,6 @@ export default function XtermPane({
   // -------------------------------------------------------------------------
   // focusTrigger: programmatic refocus from parent (palette navigation etc.)
   // -------------------------------------------------------------------------
-  const termRef = useRef<Terminal | null>(null);
-
-  // Keep termRef in sync via a second effect that runs after mount.
-  // We can't easily share the Terminal instance across the two effects without
-  // a ref, so we store it on termRef inside the mount effect instead.
-  // Actually, let's just wire focusTrigger by querying the textarea directly —
-  // that avoids the cross-effect coupling problem entirely.
   useEffect(() => {
     if (focusTrigger > 0 && containerRef.current) {
       const ta = containerRef.current.querySelector<HTMLTextAreaElement>(
@@ -244,8 +255,31 @@ export default function XtermPane({
     }
   }, [focusTrigger]);
 
-  // Suppress unused ref warning
-  void termRef;
+  // -------------------------------------------------------------------------
+  // Live font-size changes (Ctrl+Shift+= / - / 0).
+  //
+  // Setting term.options.fontSize re-rasterises glyphs immediately, but the
+  // cols/rows the terminal thinks it has are still based on the OLD cell
+  // size — so we have to fit() to recompute, refresh() to repaint, then
+  // ship the new dimensions to the PTY so bash redraws the prompt at the
+  // right width.
+  // -------------------------------------------------------------------------
+  useEffect(() => {
+    const term = termRef.current;
+    const fit = fitRef.current;
+    if (!term || !fit) return;
+    const target = fontSize ?? DEFAULT_XTERM_FONT_SIZE;
+    if (term.options.fontSize === target) return;
+    try {
+      term.options.fontSize = target;
+      fit.fit();
+      term.refresh(0, term.rows - 1);
+      const paneId = paneIdRef.current;
+      if (paneId != null) void resizePane(paneId, term.cols, term.rows);
+    } catch (e) {
+      console.warn("font-size apply failed", e);
+    }
+  }, [fontSize]);
 
   return <div ref={containerRef} style={{ width: "100%", height: "100%" }} />;
 }