The timeline opens the whole document

Making a lane one row cost the thing a row was for. A clip stopped being a row,
so there was no longer any way to open a clip and see what was inside it, and
the inside of a drawing — the most ordinary thing in the document — became
reachable only by opening it as its own tab. This is that capability back,
from the root timeline, down as far as it goes.

An expanded lane opens exactly ONE clip: the selected one. Its own keys, then
the lanes and nodes of the symbol it places, then theirs, each mapped into this
ruler by the recursive walk that was already there. Twelve clips in a lane
still cost one row, and inspection costs one branch rather than twelve.

Two things that only showed up once it ran. The portal is chosen by the whole
LINEAGE of the selection and not by the selected id: selecting a shape inside
the clip — or the end of its span — is still working inside that clip, and
matching the id alone shut the portal the instant anything under it was
touched. And selecting now waits for the pointer to come UP, because selecting
on the way down re-drew the timeline before the gesture had said anything: it
shut the portal holding the lane being dragged INTO, out from under the
pointer.

A HELD clip opens too, which the old row walk never did either. `source-time`
is nil for a hold, so the walk stopped there and the contents of every drawing
were invisible from here. Its rows are shown across the hold — which is when
the node is on screen — and marked `:unmapped?`: no keys, and no draggable
edges, because a frozen clock gives no frame inside it a place on this ruler.
Refusing to place the keys is the honest half; refusing to show the rows was
not.

Double-clicking a clip opens the symbol it places as a tab, as double-clicking
the same symbol in the pool does. That was already written and had never once
run: the track captures the pointer for a slide, so the click and double-click
that follow are delivered to the track and never to the block. The track now
resolves them itself. Fixing the delivery exposed two more: `symbol/lineage`
reported a `parent cycle` for any id in a symbol with NO nodes, because a
one-element chain is longer than zero nodes — and opening a symbol left the
selection pointing into the symbol being left, which the breadcrumb and the
inspector then tried to resolve. The editor unmounted. Both are fixed where
they were wrong, and the browser test asserts the editor is still standing
afterwards.

Audio is a clip in a lane like everything else. A dropped sound lands in one
and is trimmed and moved by the same commands; a lane holds picture or sound
and not both, which is the explicit capability the model asked for rather than
a guess per frame. The refusal lives in the commands and not only in
validation, because placement claims time: `blank` would have deleted the
sound to make room for the picture and left a perfectly valid document behind.
What is in a lane of the open symbol is drawn as a lane; what is nested inside
a placed symbol is still flattened by `audio-tracks`, so no sound is on two
rows.

Everything that enters the timeline now enters a lane: a converted take, a
symbol brought in from another project, a sound. One rule answers where —
`lane-destination` — and every symbol is born with a lane for it to answer
with. An unaimed drop fills an EMPTY lane rather than taking an occupied one
nobody pointed at, because the alternative is trimming away what was there to
make room for what was dropped.

Shift during a clip-body drag means the other intention: put this node INSIDE
the symbol the clip under the pointer places, through `nest/move-node`, which
is what keeps the world transform and the root timing. Overlap cannot say
which of the two is meant — dropping on occupied time already means claiming
it — so the person says, and a label by the pointer says it back. The label
asks `nest/move-refusal`, the same check the command makes, so it cannot
promise what the drop would refuse. Today it refuses more than it allows:
both clips have to be on screen at one frame, which two clips in one lane
never are, and a held destination has no clock to move through at all.
`docs/lane-nesting-notes.md` argues that the second refusal is stronger than
the facts require and says what would settle it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

frontend/test/browser/lane.mjs

@@ -160,6 +160,31 @@ try {
     });
     await sleep(250);
   };
+  const tabs = () => evaluate(`(() => {
+    const k = cljs.core.keyword, db = cljs.core.deref(re_frame.db.app_db);
+    return {tabs: cljs.core.clj__GT_js(cljs.core.get_in(db, [k('ui'), k('tabs')])).map(String),
+            open: String(cljs.core.clj__GT_js(cljs.core.get_in(db, [k('ui'), k('open')])))};
+  })()`);
+  // A real two-press double-click, not `.dispatchEvent`: what broke here was
+  // where the browser decides to deliver the click, which a synthetic event
+  // cannot show.
+  const doubleClick = async selector => {
+    const p = await evaluate(`(() => {
+      const el = document.querySelector(${JSON.stringify(selector)});
+      if (!el) return null;
+      const r = el.getBoundingClientRect();
+      return {x: r.left + r.width / 2, y: r.top + r.height / 2};
+    })()`);
+    assert(p, `something to double-click: ${selector}`);
+    for (const clickCount of [1, 2]) {
+      await send('Input.dispatchMouseEvent', {type: 'mousePressed', x: p.x, y: p.y,
+        button: 'left', buttons: 1, clickCount});
+      await send('Input.dispatchMouseEvent', {type: 'mouseReleased', x: p.x, y: p.y,
+        button: 'left', buttons: 0, clickCount});
+      await sleep(60);
+    }
+    await sleep(280);
+  };
   const dropPoolSymbol = async frame => {
     const points = await evaluate(`(() => {
       const source = document.querySelector('.pool-row:not(.main) .pool-item[draggable="true"]');
@@ -301,8 +326,66 @@ try {
   const lanes = Object.values(s.clip.symbols.main.nodes).filter(n => n.layout === 'sequence');
   assert.deepEqual(lanes.map(l => instances(s).filter(n => n.parent === l.id).length).sort(), [1, 3],
     'a clip body can move from one lane to another');
+
+  // EXPANDING A LANE OPENS THE SELECTED CLIP. Its own keys, and under it the
+  // lanes and nodes of the symbol it places, all on this ruler — which is what
+  // makes the whole document editable from the root timeline.
+  const rowLabels = () => evaluate(
+    `[...document.querySelectorAll('.tl-labels > .tl-label')].map(e => e.textContent.trim())`);
+  const twist = async i => {
+    assert(await evaluate(`(() => {
+      const t = document.querySelectorAll('.tl-labels > .tl-label .tl-twist')[${i}];
+      if (!t || t.disabled) return false;
+      t.click(); return true;
+    })()`), `an expander at row ${i}`);
+    await sleep(220);
+  };
+  await evaluate(`(() => { document.querySelector('.tl-track .tl-cel').click(); return true })()`);
+  await sleep(200);
+  const collapsed = await rowLabels();
+  await twist(0);
+  const opened = await rowLabels();
+  assert(opened.length > collapsed.length, 'the lane opens');
+  assert.equal(opened.filter(l => l.includes('instance')).length, 1,
+    `one clip portal, not one branch per clip: ${JSON.stringify(opened)}`);
+  const portalAt = opened.findIndex(l => l.includes('instance'));
+  await twist(portalAt);
+  const deep = await rowLabels();
+  assert(deep.length > opened.length,
+    `the portal opens the symbol the clip places: ${JSON.stringify(deep)}`);
+  // Selecting something nested must not close the portal that revealed it.
+  await evaluate(`(() => {
+    const k = cljs.core.keyword, db = cljs.core.deref(re_frame.db.app_db);
+    const sel = cljs.core.get_in(db, [k('ui'), k('selection')]);
+    const path = cljs.core.nth(sel, 3);
+    re_frame.core.dispatch(cljs.core.vector(
+      k('arthur.events.ui/select'),
+      cljs.core.vector(k('node'), cljs.core.nth(sel, 1), cljs.core.nth(sel, 2),
+                       cljs.core.conj(path, k('made-up-child')))));
+    return true;
+  })()`);
+  await sleep(220);
+  assert.equal((await rowLabels()).filter(l => l.includes('instance')).length, 1,
+    'a selection under the clip keeps its portal open');
+  await twist(portalAt);
+  await twist(0);
+
+  const before = await tabs();
+  const tabChips = () => evaluate('document.querySelectorAll(".tabs .tab").length');
+  const chipsBefore = await tabChips();
+  await doubleClick('.tl-track .tl-cel');
+  const after = await tabs();
+  assert.equal(after.tabs.length, before.tabs.length + 1,
+    `double-clicking a clip opens the symbol it places, as the pool row does: ${JSON.stringify(after)}`);
+  assert(!before.tabs.includes(after.open) && after.tabs.includes(after.open),
+    `the opened symbol is the one in front: ${JSON.stringify(after)}`);
+  assert.equal(await tabChips(), chipsBefore + 1,
+    'the opened symbol is drawn as one more tab');
+  assert.equal(await evaluate('document.querySelectorAll("#app > *").length'), 1,
+    'opening from the timeline leaves the editor standing: a stale node selection ' +
+    'pointing into the symbol just left used to throw and unmount it');
   assert.equal(errors.length, 0, JSON.stringify(errors));
-  console.log('PASS: generic lanes preview, rename, move, place, trim, roll, and ripple clips');
+  console.log('PASS: generic lanes preview, rename, move, place, open, trim, roll, and ripple clips');
 } finally {
   if (ws?.readyState === WebSocket.OPEN) {
     ws.send(JSON.stringify({ id: 999999, method: 'Browser.close' }));