Make preserved-identity DockFlip moves clip-safe and immediate

Context

The visual verification pass on #15099 / PR #15133 separated two defects that looked like one flicker:

1. the projection transaction broke silent=true inside Neo.tab.Container.removeAt(), allowing tab titles to change before the common-parent body move;
2. after repairing that ownership breach, one brief blank-looking frame remains at the start of the FLIP inversion.

The first defect belongs to PR #15133. This ticket isolates the remaining motion problem instead of expanding that PR into a shared animation redesign.

Empirical evidence from the repaired tree:

• three independent live runs sampled 979–997 requestAnimationFrame frames with zero active-header/empty-body intervals and zero page errors;
• destination content was mounted and coherent before DockFlip.play() began;
• sequential playback still showed one frame where the transformed pane was outside the destination tab body's visible clip;
• the same-identity path also spent roughly 116–141 ms in stage A waiting for captured markers to detach, even though atomic DOM permanence deliberately keeps those exact nodes connected.

The Problem

Neo.main.addon.DockFlip currently models a projection swap as outgoing marker nodes detaching and replacement marker nodes appearing. Neo's native atomic cross-parent move has a second, stronger reality: the same pane DOM nodes survive and are reparented.

On that preserved-identity path:

• stage A burns several frames waiting for a detach that must not happen;
• inversion applies the old cross-parent geometry to an element already inside its destination .neo-tab-body-container;
• that body owns overflow: hidden, so the transformed pane can sit outside its final clip for a frame while its header remains visible.

The document and pane identities are correct; the presentation briefly tells a false story.

The Architectural Reality

• src/main/addon/DockFlip.mjs#captureFirst/play owns presentation-only First/Last/Invert/Play motion.
• src/container/Base.mjs#insert and the tab/body adapters own same-window DOM permanence through silent source removal plus one closest-common-parent update.
• resources/scss/src/tab/Container.scss intentionally clips tab bodies; globally disabling that clip is not a safe motion solution.
• src/dashboard/DockMotionSignal.mjs remains the sole counted lifecycle signal.
• Closed #14929 established FLIP as the committed-operation motion layer. This ticket repairs its preserved-identity path; it does not move authority into Demo C.

The Fix

Teach DockFlip to distinguish replacement-node swaps from preserved-identity cross-parent moves.

1. If the post-projection marker set is exactly the captured connected identities, bypass the detach wait and measure Last immediately.
2. Give cross-clipping moves a presentation path that remains visible from First to Last. Prefer a host-owned overlay/staging projection or another bounded mechanism that preserves the real tab-body clip; do not make every tab body globally overflow-visible.
3. Retain the current direct transform path for moves whose First geometry remains visible within the destination clipping context.
4. Keep entering/replacement-node behavior, reduced-motion collapse, token timing, and DockMotionSignal ownership unchanged.
5. Restore every temporary node/style on success, rejection, interruption, and destroy.

Contract Ledger

Decision Record impact

none — this is aligned with the presentation-only FLIP contract from closed #14929 and the native atomic-move contract.

Acceptance Criteria

• A preserved marker set skips stage-A detach polling; a replacement marker set retains the bounded wait.
• Split and return operations expose no painted frame where a visible active tab header has fully clipped/blank pane content.
• The moved pane component id and DOM node identity remain unchanged.
• Normal tab-body overflow: hidden remains unchanged before, during, and after motion unless a narrower host-owned staging layer proves necessary.
• No global overflow: visible selector is introduced.
• Reduced motion lands instantly with no temporary node, style, class, timer, or signal residue.
• Success, rejection, destroy, and post-invert failure paths restore all temporary presentation state.
• Focused DockFlip unit coverage pins both identity branches and cleanup.
• Mounted Demo C evidence includes a sequential-frame oracle for the split and return beats, not only DOM-presence checks.

Out of Scope

Changing dock documents or reducers; replacing Demo C's tab chrome (owned by #15136); generic keyed VDOM reconciliation; cross-window choreography; changing the motion vocabulary.

Avoided Traps

• Global overflow override: a live-only experiment interfered with the tour completion contract and changes normal tab semantics.
• Calling this a video-only artifact: random-access WebM seeking amplified the symptom, but sequential playback retained the one clipped motion frame.
• Changing the transfer again: repaired live traces prove content is coherent before DockFlip starts.
• Using a screenplay delay: timing cannot repair an invalid clipping geometry.

Related

Related: #13158 · #14929 · #15099 · #15136 · PR #15133

Live latest-open sweep: checked latest 20 open issues at 2026-07-13T21:27:12.827Z; no equivalent found. The nearest ticket, #15136, explicitly excludes FLIP geometry motion. A2A all-state recency sweep: latest 30 messages checked immediately before creation; no overlapping claim or intent.

Origin Session ID: f95e01ff-ba36-409a-98af-573263fab247

Retrieval Hint: DockFlip preserved identity connected markers tab body clipping blank frame stage A detach wait