PR #15555 - manaflow-ai/cmux

Tab drag session for cmux-next (plans/cmux-next/REWRITE.md "Tab drag"): one drag, every destination.

What changes

TabDragSession (CmuxNextApp/Drag) replaces the first cut. One borderless non-activating panel carries the ghost across every window and outside them. Over a strip it folds into an inline tab, and the strip opens a gap. Everywhere else it unfolds into a Liquid Glass card with a live terminal or browser snapshot. The ghost follows the pointer with no lag. Every discontinuity springs: card to tab, slot change, spring back, drop.
The session asks every TabDropTargetProviding in every window by screen point, in this order: sidebar, strips, layout. The sidebar and the layout get adapters (TabDropTargets.swift). A pane center maps to "join that pane's strip".
Each outcome commits ONE daemon command with a client transaction and an optimistic patch: move-tab, tab-to-new-split, tab-to-new-column, tab-to-new-workspace (placed at the sidebar slot or group), move-tab-to-workspace. Release outside every window tears off: a new workspace opens in a new window whose strip lands under the pointer, and WindowManager persists the window. A drag that carries the whole workspace moves its window instead. Escape, no target, or a rejected command springs the tab back.
Whole-group drags use the tab-groups-v1 group moves.
TabDragLifecycle makes sure every drag ends exactly once. It calls restoreDetachedTab/restoreDetachedGroup unless the transaction settled.
Auto-scroll: strips and the sidebar already scroll themselves. The new LayoutRootView.autoscrollTabDrag scrolls niri columns near the edges. This is a small additive API in CmuxNextLayout.
The display link runs only while the ghost animates or columns auto-scroll, and stops when the drag ends. Measured idle CPU is 0.0%.

Fixes

Fallback split on the left or top edge failed with swap-pane: unknown pane/target on daemons without tab-drag-v1. Reproduced against a live cmux-tui: dragging a pane's only tab onto its own left edge moves the tab into the new pane, the source pane closes, and the swap targets a pane that no longer exists. The resolver now refuses that no-op drop. The fallback swaps only when the target pane still exists, and uses split {tab} when the daemon honors it.
Same-pane move-tab sent the final index, but the daemon expects pre-move insertion coordinates. Moving a tab one slot right did nothing, and longer moves landed one slot short. TabMoveIndex converts the index.

Verification

swift build --build-tests passes. swift test passes for the whole package. The new TabDragTests cover outcome resolution, cancel and restore invariants, transaction settle, ghost motion, index conversion, and tear-off geometry. check-no-godfiles.sh passes.
Fleet build tabdrag-v1 at d2e6984 was launched locally with a clean environment and CMUX_NEXT_NO_ACTIVATE=1. Through the control socket, tab move-right by one slot now moves the tab; before this fix it was a no-op. With 5 tabs and 2 panes, the app idles at 0.0% CPU and 117 MB RSS.
UNVERIFIED: the drag itself (ghost, drop targets, tear-off, spring-load, auto-scroll). The app ignored mouse events posted to its pid while inactive, so a background click did not reach the strip. cmux-cua onboarding is not complete, and a foreground drag would take over the user's display. No recording or mid-drag screenshot was taken.

Known limits

Dropping in front of the first column is refused: no daemon command can express it.
A strip drop that changes group membership sends a second, group command.
With the current daemon (no tab-drag-v1), a new split or column first spawns a PTY, moves the tab in, and then closes that PTY.

Changelog

none

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Summary by cubic

Replaces the first tab-drag cut with a full TabDragSession: one ghost panel follows the pointer across every window and outside them, and every drop outcome commits exactly one daemon command.

Drag session

The ghost folds into an inline tab over strips (opening a gap) and unfolds into a Liquid Glass preview card with a live thumbnail elsewhere.
Every surface in every window is hit-tested by screen point, in priority order: sidebar, strips, layout; whole-group drags use tab-groups-v1 moves, and releases outside every window tear off into a new window under the pointer, or move the source window when the drag carries the whole workspace.
Each outcome sends one command with a client transaction and optimistic patch; TabDragLifecycle guarantees every drag ends exactly once and restores the source tab unless the transaction settled.
The new LayoutRootView.autoscrollTabDrag scrolls niri columns near the edges; the display link runs only while the ghost animates or a screen auto-scrolls.

Fixes

Same-pane move-tab now converts final indices to the daemon's pre-move insertion coordinates (a one-slot move right was a no-op, longer moves landed a slot short).
Fallback split on left/top edges no longer swaps with a pane that moving the pane's only tab closed (unknown pane/target); the resolver also refuses splitting the source pane with its last tab.