Keep a grip from laying out before it has a size

The gate of a5184b3 was red: three tests in variable-rows.test.ts
failed with "document is not defined". setGrabbable laid the grip
out right away, so a row that had not been sized yet squeezed its
label to nothing. The label then measured its text, and Node has no
canvas for that.

- setGrabbable only lays out again once the grip has a width. The
  first resize places the mark with the dots' inset anyway.

Tests:
- variable-rows + block-drag: 2 files, 13 tests, all green
  (variable-rows was 3 failed / 5 passed before)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/ui/widgets/Grip.ts

@@ -43,7 +43,9 @@ export class Grip extends Widget {
   setGrabbable(on: boolean): void {
     if (this.grabbable === on) return;
     this.grabbable = on;
-    this.layout();
+    // Not laid out yet: the first resize places the mark, and squeezing it to nothing now would only
+    // make its label measure itself for no use.
+    if (this.w > 0) this.layout();
     this.invalidate();
   }