Place assets by dragging them from the panel into the view

The context menu entry is gone; dragging replaces it entirely. The drop
target is the view under the pointer, not the active tab, so the same
gesture works for scenes and for container templates.

While dragging, an outline shows the footprint the object will have plus
a crosshair on its origin, turning red when the drop would be refused.
Preview and command share one function (placementRejection), so the
outline cannot promise something the drop then rejects.

A left-press used to select the list row immediately. Since the container
tab follows the selection, it switched to the very container being dragged
and every drop would have nested it inside itself. The list now selects on
release without a drag: picking something up is not the same as choosing it.

tools/e2e/04-assets.mjs

@@ -1,15 +1,75 @@
-/** The asset panel: placing from the context menu, creating through "+", and resetting an asset value. */
+/** The asset panel: placing by dragging into the view, creating through "+", and resetting an asset value. */
 export async function assets(t) {
-  const { check, sleep, evaluate, json, shot, click, clickText, undo, select, objects, asset } = t;
+  const {
+    check,
+    sleep,
+    evaluate,
+    json,
+    shot,
+    click,
+    clickText,
+    mouse,
+    press,
+    redo,
+    undo,
+    select,
+    objects,
+    asset,
+  } = t;
 
-  // 8. Context menu on the Crate asset → menu shows "Place in scene".
+  // 8. Context menu on the Crate asset. Placing left the menu when it became a drag.
   await clickText('Crate', 'right');
-  check('asset context menu opens', Boolean(await evaluate(`__gm.locate('Place in scene')`)));
+  check('asset context menu opens', Boolean(await evaluate(`__gm.locate('Delete…')`)));
+  check('placing is no longer a menu item', !(await evaluate(`__gm.locate('Place in scene')`)));
   await shot('11-context-menu');
-  await clickText('Place in scene');
-  await sleep(100);
+  await press('Escape');
+  await sleep(150);
+
+  // 8b. Drag the Crate asset from the panel into the scene view. Driven step by step rather than
+  //     with `drag()`, because the preview only exists while the button is still down.
+  const from = await json("__gm.assetRow('crate')");
+  const centre = await json('__gm.viewportCenter()');
+  const to = { x: centre.x + 70, y: centre.y - 50 };
+  await mouse('mouseMoved', from.x, from.y);
+  await mouse('mousePressed', from.x, from.y);
+  for (let i = 1; i <= 8; i++) {
+    const x = from.x + ((to.x - from.x) * i) / 8;
+    const y = from.y + ((to.y - from.y) * i) / 8;
+    await mouse('mouseMoved', x, y, 'left', { buttons: 1 });
+    await sleep(16);
+  }
+  const preview = await json('__gm.dropPreview()');
+  check(
+    'dragging an asset over the view previews where it would land',
+    Boolean(preview) && preview.assetId === 'crate' && preview.rejection === '',
+    JSON.stringify(preview),
+  );
+  await shot('11b-drop-preview');
+  await mouse('mouseReleased', to.x, to.y);
+  await sleep(200);
+
   const count = (await objects()).length;
-  check('placing from the menu adds an object', count === 8, `objects=${count}`);
+  check('dropping the asset adds an object', count === 8, `objects=${count}`);
+  const placed = await json(
+    '(() => { const id = __gm.state.selectedObjectIds[0]; return __gm.sceneObjects().find(o => o.id === id) ?? null; })()',
+  );
+  // The preview is a promise: whatever it drew has to be where the object then sits, or the
+  // outline was decoration rather than a prediction.
+  check(
+    'the object lands where the preview promised, and is selected',
+    Boolean(placed) &&
+      placed.assetId === 'crate' &&
+      placed.position.x === preview.x &&
+      placed.position.y === preview.y,
+    JSON.stringify({ placed: placed && placed.position, preview }),
+  );
+  check('the preview is gone once the asset is placed', (await json('__gm.dropPreview()')) === null);
+  await undo();
+  const afterUndo = (await objects()).length;
+  check('one undo takes the dropped object back out', afterUndo === 7, `objects=${afterUndo}`);
+  await redo();
+  await sleep(150);
+  check('redo puts it back', (await objects()).length === 8);
 
   // 9. Create a new asset through the + menu.
   await clickText('+');