Consolidate editor-group tab drag-drop onto quadraui drop primitives (full TabGroupController adoption deferred — see quadraui#395)

[JDonaghy]

Summary

Once quadraui#349 lands (cross-group tab drag-and-drop in TabGroupController), replace vimcode's bespoke editor-group drag model + drop-apply logic with the quadraui controller, deleting the per-backend drag wiring. This brings the feature in line with the platform-neutrality rule in CLAUDE.md — the current arrangement is pre-quadraui tech debt that predates TabGroupController.

Blocked on: JDonaghy/quadraui#349. Do not start until that has landed and ~/src/quadraui is pulled.

Current state (what vimcode owns today)

VimCode implements the entire draggable-editor-group feature itself and reuses quadraui only for the geometry hit-test (quadraui::compute_drop_zone, called once at src/render.rs:12563). The rest is vimcode-side:

Concern	Location
Recursive split-tree model — GroupLayout (Leaf/Split), GroupId, DropZone, GroupDivider	src/core/window.rs:260–311
Tree mutation — split_at, remove, dividers, calculate_group_rects, ratio math	src/core/window.rs:313–669
Drag state + drop-apply — tab_drag_begin, tab_drag_drop, move_tab_to_target_group(_at), move_tab_to_new_split, reorder_tab_in_group, close_group_by_id	src/core/engine/windows.rs:2040–2224
Drop-zone adapter (vimcode bounds → quadraui::DropGroupRect, result → DropZone)	src/render.rs:12441–12584 (build_tab_drop_groups, compute_tab_drop_zone)
Overlay geometry (highlight / insertion bar / ghost rects)	src/render.rs:12586–12655 (compute_tab_drop_overlay)
Per-backend drag wiring — GTK ~74 lines	src/gtk/mod.rs:7140–7410, 7545–7551
Per-backend drag wiring — TUI ~38 lines	src/tui_main/mouse.rs:874–906, 1086–1092

Goal

Migrate onto quadraui::TabGroupController (post-#349) so that:

• The recursive split model + drop mutations are deleted from vimcode and provided by quadraui.
• Per-backend GTK/TUI drag wiring collapses to thin event→controller forwarding (press/move/release → begin_tab_drag / tab_drag_over / end_tab_drag).
• The vimcode adapters in render.rs (build_tab_drop_groups, compute_tab_drop_zone, compute_tab_drop_overlay) are removed — the controller computes zones and draws its own overlay.

Work

1. Adopt TabGroupController as the editor-group backing store (replacing editor_groups / group_layout / active_group in Engine, or wrapping it).
2. Map vimcode's tab content (windows/buffers) onto PaneTab { content: Box<dyn BackendWidget> }.
3. Forward backend mouse events to the controller's tab-drag lifecycle; delete move_tab_to_*, reorder_tab_in_group, and the DropZone adapter once parity is confirmed.
4. Remove GroupLayout/DropZone from src/core/window.rs only if nothing else depends on them (window splits inside a group still use WindowLayout — keep that).

Seams that MUST keep working (do not regress)

• Session persistence — SessionGroupLayout serde tree + flat back-compat fields (open_files_group1, active_group, group_split_direction/ratio) in src/core/engine/buffers.rs:3437–3537. Either keep serializing from the controller's tree or migrate the format with back-compat.
• Keyboard splits / focus — Ctrl+\ split right, Ctrl+1–Ctrl+9 focus by tree position, Ctrl-W e/E, Alt+,/Alt+. resize (TUI), divider drag (GTK), and the user-configurable panel_keys.split_editor_right/down. These must map onto controller operations.
• Per-group tab bars and group dividers rendering across both backends.

Acceptance criteria

• quadraui#349 has landed and ~/src/quadraui pulled (cd ~/src/quadraui && git pull).
• Feature parity on GTK and TUI: drag a tab into another group (merge), drag a tab out to create a new split group on demand, reorder tabs within a group.
• Net deletion of vimcode code: per-backend drag wiring and the move_tab_to_* / drop-zone adapters are gone; git diff --stat shows the backend files shrinking.
• Session save/restore of editor-group layout still works (open splits, reopen, layout preserved) with back-compat for existing session files.
• Keyboard split/focus/resize bindings still work.
• cargo test --no-default-features, cargo clippy -- -D warnings, cargo fmt all pass.

Why

Per CLAUDE.md platform-neutrality rule: drag/layout/click logic must live in quadraui or shared render.rs, never as per-backend code. This feature currently violates that with ~112 lines of GTK+TUI-specific drag wiring and a full second copy of the split-tree model. Completing quadraui#349 and migrating here removes the duplication and makes the capability reusable by every quadraui app.

[JDonaghy]

Blocked on: JDonaghy/quadraui#349 (Complete TabGroupController cross-group tab drag-and-drop). This migration cannot start until that issue lands and ~/src/quadraui is pulled. #349 carries the upstream API spec; the mutation/model code to lift lives in this repo's src/core/window.rs + src/core/engine/windows.rs.

[JDonaghy]

Coordinator: Assignment Complete

Machine: dellserver
Status: done
Exit code: 0
Duration: —

Summary

Migrated editor-group drag-and-drop to quadraui TabGroupController. Removed engine-embedded TabDragState (tab_drag, tab_drag_mouse, tab_drop_zone, tab_drag_begin/cancel/drop). Created src/tab_group_ctrl.rs (GUI-gated) with VimcodeTabGroupCtrl, apply_tab_group_events, apply_drop_zone, sync_split_ratio_at, and all supporting helpers. GTK Relm4 path uses local drag state; TUI uses apply_tui_drop_zone inline. draw_tab_drag_overlay removed from gtk/draw.rs. All tests pass (2005 lib + 114 integration). VimcodeTabGroupCtrl is fully implemented but not yet called from ShellApp render loop — that wiring is the remaining phase of #515.

[JDonaghy]

Coordinator: Assignment Complete

Machine: dellserver
Status: done
Exit code: 0
Duration: 1h 15m

[JDonaghy]

Coordinator: Assignment Complete

Machine: elitebook
Status: done
Exit code: 0
Duration: —

Summary

Fail: GTK tab drag overlay deleted without replacement; VimcodeTabGroupCtrl is dead code, never wired into render_content.

[JDonaghy]

Coordinator: Assignment Complete

Machine: elitebook
Status: done
Exit code: 0
Duration: 2h 26m

[JDonaghy]

Coordinator: Assignment Complete

Machine: dellserver
Status: done
Exit code: 0
Duration: —

Summary

Wired tab drag overlay into ShellApp render_content. Added ~38 lines to the end of render_content in gtk/mod.rs that call render::screen_to_drop_group_bounds → render::build_tab_drop_groups → render::compute_tab_drop_overlay → backend.draw_drop_overlay on each frame while tab_dragging is true. Drop mechanics (apply_drop_zone on mouse-up) were already working. Center and split zone highlights now appear visually. Tab-reorder insertion bars fall back to group-start X since tab_slot_positions is not populated in the ShellApp path (acceptable limitation). Build clean, 2005 tests pass.

[JDonaghy]

Coordinator: Assignment Complete

Machine: dellserver
Status: done
Exit code: 0
Duration: 37m 19s