Chromatic: snap pane splits to final geometry instead of tweening

lib/.storybook/preview.ts

@@ -29,6 +29,12 @@ if (window?.navigator?.userAgent?.includes('Chromatic')) {
   // A blinking cursor is captured on-or-off depending on the frame; freeze it to a
   // stable solid block so the terminal contributes no non-determinism.
   cfg.terminal.cursorBlink = false;
+  // Snap pane splits/restores to their final geometry instead of tweening. A
+  // mid-tween split resizes panes through many transient widths, and xterm's DOM
+  // renderer can latch a frame where a pane is still narrow — freezing its last
+  // line clipped (`user@dormouse:~$` → `user@do`) even after the layout settles.
+  // Instant geometry removes that race at the source.
+  cfg.layout.animate = false;
 }
 
 // Collect all CSS variable names across all themes for cleanup

lib/src/cfg.ts

@@ -34,4 +34,13 @@ export const cfg = {
      *  stable solid block rather than being captured mid-blink (non-deterministic). */
     cursorBlink: true,
   },
+  layout: {
+    /** When false, Lath pane geometry changes (split / restore / kill / drag) apply
+     *  instantly with no tween. Disabled under Chromatic: a mid-tween split resizes
+     *  panes through many transient widths (briefly near-zero), and xterm's DOM
+     *  renderer can latch onto one of those frames and leave a pane painted blank or
+     *  clipped (`user@dormouse:~$` → `user@do`) even after the geometry settles.
+     *  Snapping straight to the final geometry removes that whole race. */
+    animate: true,
+  },
 };

lib/src/components/wall/lath-wall-engine.ts

@@ -24,6 +24,7 @@ import {
   createAnimator,
 } from '../../lib/lath/animator';
 import { prefersReducedMotion } from '../../lib/ui-geometry';
+import { cfg } from '../../cfg';
 import { UNNAMED_PANEL_TITLE } from '../../lib/terminal-registry';
 import type { ResolvedSplitDirection as DorResolvedSplitDirection } from 'dor/commands/types';
 import {
@@ -163,9 +164,11 @@ export function createLathWallEngine(
 ): LathWallEngine {
   const snapshot = (): LathWallSnapshot => store.getSnapshot();
 
-  // 0 under reduced motion, so entry/exit/tween all collapse to instant through the
-  // very same code path. Tests inject a fixed duration (or 0) via `opts`.
-  const durationMs = opts?.durationMs ?? (prefersReducedMotion() ? 0 : LATH_MOTION_MS);
+  // 0 under reduced motion (or when layout animation is disabled, e.g. Chromatic),
+  // so entry/exit/tween all collapse to instant through the very same code path.
+  // Tests inject a fixed duration (or 0) via `opts`.
+  const durationMs =
+    opts?.durationMs ?? (!cfg.layout.animate || prefersReducedMotion() ? 0 : LATH_MOTION_MS);
   const animator = createAnimator({ durationMs, easing: LATH_EASING });
 
   // Presentation-only side state (never in the store snapshot): the frame/wake