Implement dragging actions for vertical tab groups

[johnturcoo]

Description

Adds drag-and-drop reordering for vertical tab groups. Users can drag a group to reorder it as a block. Single tabs can also be dragged into and out of an expanded group.

Changes

New actions (app/src/workspace/action.rs): StartGroupDrag, DragGroup, DropGroup

Group drag handler (app/src/workspace/view.rs):

• on_group_drag — block reorder via move_group_block when the dragged header's midpoint crosses an above/below neighbor's midpoint
• target_group_at_y — hit-tests group container rects (with a 6px edge margin so the ungrouped zone between adjacent groups is reachable)
• neighbor_drag_rect — substitutes the group container's rect for stale per-tab rects when a neighbor lives inside a collapsed group.
• assign_tab_to_group — flips tabs[i].group_id for cross-group transitions during a tab drag

Nested-Draggable support (crates/warpui_core/src/elements/drag/draggable.rs, crates/warpui_core/src/presenter.rs):

• New Draggable::with_defer_to_handled_child_mouse_down() builder. When the flag is set, the Draggable yields its mouse-down to any descendant Draggable that already claimed the event.
• New EventContext::descendant_draggable_initiated flag — set unconditionally when a Draggable claims a mouse-down, read only by Draggables that opt into deferral.
• Backward-compatible: existing Draggables don't opt in, so existing drag behavior is unaffected.
• This allows proper handling of nested draggable elements

Linked Issue

https://linear.app/warpdotdev/issue/APP-4619/vertical-tab-group-dragging-support

Testing

• I have manually tested my changes locally with ./script/run

Screenshots / Videos

Demo video

[oz-for-oss]

@johnturcoo

I'm starting a first review of this pull request.

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[oz-for-oss]

Overview

This PR adds vertical tab group drag-and-drop support plus nested-draggable deferral plumbing. I found two blocking correctness issues in the tab/group drag behavior. No security-specific findings were identified, and the provided spec context did not include approved requirements to compare against.

Concerns

• Dragging a tab into a non-adjacent expanded group only flips group_id and returns, while the renderer only groups consecutive tabs. This can split one logical group into multiple rendered containers and leave later group-block operations working on an invalid range.
• Skipping the per-tab draggable for a sole group member means the only tab in a group cannot be dragged out to dissolve/prune that group.

Verdict

Found: 0 critical, 2 important, 0 suggestions

Request changes

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[peicodes]

Can we verify what the behavior is when cross-window tab dragging is enabled? Two things I wanna test:

• Does dragging a tab out into a new window still work?
• Does dragging a tab from one window to another still work?
• What happens if I try to drag a group out?
• What happens if I try to drag a tab within a group out?
• What happens if I try to drag a tab from one window into another window's tab group?

[johnturcoo]

@peicodes To answer your questions:

• Does dragging a tab out into a new window still work?
  Yes
• Does dragging a tab from one window to another still work?
  Yes
• What happens if I try to drag a group out?
  Currently you can not drag a group out.
• What happens if I try to drag a tab within a group out?
  Works as expected, the group does not update until tab is released, but this should be a minor fix.
• What happens if I try to drag a tab from one window into another window's tab group?
  Currently this does not work, it can only be dropped into the vertical tab pane outside of groups.

I don't think any of these are blocking, IMO. I want to land this by eod so it can be on dogfood tmw for feedback. I will address these bugs as fast follows.

Loom for reference: https://www.loom.com/share/ccb7253b50c74e2f95a7b9f8ccbfef09