Move, turn and scale an item in the composition picture live

Dragging an object in the Composition tab's picture writes the copy
of the composition at the playhead and asks for a preview once a
frame. The tab answered every such preview by building that copy
again from the composition, which still held the old value. So the
object stood still while dragged, and a move often did nothing at
all, because its end was read from a copy that had just been rebuilt.
Values typed in the inspector were not affected.

- SceneViewport gets editing(), which says whether a gesture of this
  view is under way: a drag, a size or turn handle, a polygon corner,
  a joint anchor or a tilemap stroke.
- CompositionCanvas leaves its copy alone while such a gesture sends
  its frame previews. The command at the end writes the composition
  as before, and the copy is built again from it.

Tests:
- tsc, eslint and Prettier are clean.
- e2e 69 on melektaus-atx: ALL PASSED, 16 checks, 69l is new. An item
  dragged 60 px to the right stands at +60 px while the button is
  still down (590 to 650) and has x = 60 in the model afterwards.
- Counter-check: rebuilding the copy on every preview again turns 69l
  red. The item stays put while dragged and x is 0 after letting go.
- Gate on melektaus-atx: green, 305 files, 4714 tests.
- Turning and scaling by the handles were not tried in the browser.
  They take the same path and editing() covers them.

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

tools/e2e/69-composition-tab.mjs

@@ -6,7 +6,8 @@
  * takes an item with its object, and Play runs the playhead to the end. The video of an item about
  * to begin is opened ahead, so the cut to it shows its picture at once, and Play waits for a video
  * whose player is still loading. A copied item is pasted under the pointer or at the playhead. A
- * video dragged in wears a sprite the composition keeps to itself.
+ * video dragged in wears a sprite the composition keeps to itself. An item dragged in the picture
+ * follows the pointer while the button is down.
  *
  * Everything it adds carries a `ct69` id and goes again at the end.
  */
@@ -348,6 +349,34 @@ export async function compositionTab(t) {
     JSON.stringify({ videoDrop, one, row, two, undone }),
   );
 
+  // 69l. An item dragged in the picture follows the pointer while the button is down, not only when
+  // it is let go: the drag's preview each frame does not build the picture again from the document.
+  await evaluate('__gm.composition.scrub(15)');
+  await sleep(100);
+  const front = (await json('__gm.composition.shown()')).at(-1)?.id ?? '';
+  const startAt = await json(`__gm.composition.objectScreen('${front}')`);
+  const objectX = () =>
+    json(
+      `__gm.state.project.assets.find((a) => a.id === '${CUT}').objects.find((o) => o.id === '${front}').position.x`,
+    );
+  const xBefore = await objectX();
+  await mouse('mouseMoved', startAt.x, startAt.y);
+  await mouse('mousePressed', startAt.x, startAt.y);
+  for (let i = 1; i <= 6; i++) {
+    await mouse('mouseMoved', startAt.x + i * 10, startAt.y, 'left', { buttons: 1 });
+    await sleep(16);
+  }
+  await sleep(100);
+  const during = await json(`__gm.composition.objectScreen('${front}')`);
+  await mouse('mouseReleased', startAt.x + 60, startAt.y);
+  await sleep(150);
+  const xAfter = await objectX();
+  check(
+    'an item dragged in the picture follows the pointer while dragged and keeps the place let go',
+    Math.abs(during.x - startAt.x - 60) < 2 && xAfter > xBefore,
+    JSON.stringify({ front, startAt, during, xBefore, xAfter }),
+  );
+
   await showTab('scene');
   await mutate(`
     const p = __gm.state.project;