linux: implement split pane actions, surface close, and input fixes

[Jesssullivan]

Summary

• Implement split pane management actions: new split, goto split, resize split, equalize splits, toggle split zoom (stub)
• Add move_tab action for keybind-driven tab reordering via AdwTabView
• Fix split tree update on surface close (shell exit / close-surface callback) — promotes sibling node and rebuilds widget tree
• Add consumed_mods logic for shift key and Num Lock modifier translation
• Define GDK_MOD2_MASK locally (C macro not visible through Zig's @cImport)

Greptile findings addressed

• P1: Use-after-free in applyRatiosIdle — carry workspace ID instead of raw pointer
• P1: Orphaned panel when splitPane fails — find target node before creating panel, clean up on error
• P1: buildWidget called before leaf widgets are unparented — close old tab page and detach leaves from old GtkPaneds first
• P1: Resize delta uses hardcoded 1000px divisor — use actual pane size from gtk_widget_get_width/height
• P2: toggle_split_zoom returned true before implemented — changed to return false
• P2: collectLeaves silently drops leaves on OOM — added log.warn

Test plan

• CI passes on all distros (Arch, Debian, Ubuntu, Fedora, Rocky)
• Nix flake check passes
• Manual smoke test on honey: split pane, navigate, resize, close pane, close surface

[greptile-apps]

Greptile Summary

This PR implements the full split pane action surface for the Linux client: new_split, goto_split, resize_split, equalize_splits, and move_tab. It also fixes the previously-flagged P1s — orphaned panel on split failure, use-after-free in the idle ratio callback, buildWidget before unparenting leaves, and hardcoded 1000 px divisor — plus adds consumed_mods Shift tracking and Num Lock translation.

• P1 – focused_panel_id stale after surface close: removePanel (unlike detachPanel) never updates focused_panel_id. After a pane closes via shell exit, the workspace's focused ID still points to the dead panel; every subsequent new_split / goto_split / resize_split silently returns false until the user triggers a GTK focus event by clicking a pane.

Confidence Score: 4/5

Safe to merge after addressing the stale focused_panel_id bug; all other previously-flagged P1s are resolved.

One new P1 found: after a pane closes via surface exit, focused_panel_id is not updated, leaving the workspace unable to perform any further split actions without a manual click. All previously-flagged P1s (use-after-free, orphaned panel, wrong divisor, buildWidget ordering) are correctly addressed. The remaining findings are P2 (OOM leak in splitPane, incomplete consumed_mods for symbols).

cmux-linux/src/app.zig — onCloseSurface split path needs focused_panel_id update after closePane

Important Files Changed

Filename	Overview
cmux-linux/src/app.zig	Implements six new split pane actions, move_tab reordering, and a rebuildWorkspaceWidget helper; one P1 remains: focused_panel_id is not updated after surface close, breaking all subsequent split actions on the surviving pane.
cmux-linux/src/split_tree.zig	Adds collectLeaves, adjacentLeaf, equalize, and applyRatios; correctness is solid except for a minor memory leak in splitPane when createLeaf OOMs after the first node is allocated.
cmux-linux/src/surface.zig	Adds consumed_mods Shift detection and GDK_MOD2_MASK Num Lock translation; Shift marking is incomplete for symbol keys (Shift+1 → ! etc.) but is a clear improvement over always-NONE.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[onAction] --> B{Action type}
    B -->|NEW_SPLIT| NS[handleNewSplit]
    B -->|GOTO_SPLIT| GS[handleGotoSplit]
    B -->|RESIZE_SPLIT| RS[handleResizeSplit]
    B -->|EQUALIZE_SPLITS| ES[handleEqualizeSplits]
    B -->|TOGGLE_SPLIT_ZOOM| TZ[handleToggleSplitZoom no-op]
    B -->|MOVE_TAB| MT[handleMoveTab]
    NS --> NS1[findNodeByPanel before create]
    NS1 --> NS2[createTerminalPanel]
    NS2 --> NS3[split_tree.splitPane]
    NS3 -->|error| NS4[removePanel + restore focused_id]
    NS3 -->|ok| NS5[rebuildWorkspaceWidget]
    NS5 --> NS6[gtk_widget_grab_focus]
    GS --> GS1[adjacentLeaf]
    GS1 --> GS2[ws.focused_panel_id = leaf.panel_id]
    GS2 --> GS3[gtk_widget_grab_focus]
    RS --> RS1[findResizeSplit]
    RS1 --> RS2[delta = amount / paned_size]
    RS2 --> RS3[clamp ratio 0.1 to 0.9]
    RS3 --> RS4[applyRatios idle callback carries ws_id]
    MT --> MT1[adw_tab_view_reorder_page]
    MT1 --> MT2[tm.selected_index = target_idx]
    onClose[onCloseSurface] --> OC1{leafCount > 1?}
    OC1 -->|yes| OC2[closePane + removePanel focused_panel_id NOT updated]
    OC2 --> OC3[rebuildWorkspaceWidget]
    OC1 -->|no| OC4[removePanel close workspace if empty]
    RBW[rebuildWorkspaceWidget] --> R1[close old AdwTabPage]
    R1 --> R2[detachLeavesFromParents]
    R2 --> R3[split_tree.buildWidget]
    R3 --> R4[adw_tab_view_insert]
    R4 --> R5[g_idle_add applyRatiosIdle by ws_id]

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_{Reviews (6): Last reviewed commit: "fix: grab GTK focus on new pane after sp..." | Re-trigger Greptile}

[greptile-apps]

Stale focused_panel_id after pane close

removePanel removes the panel from both maps but never updates focused_panel_id (confirmed — only detachPanel does that). After this path returns, ws.focused_panel_id still equals the now-dead panel_id. On the next handleNewSplit / handleGotoSplit / handleResizeSplit, findNodeByPanel(root, focused_id) returns null and every action silently returns false. The user is left unable to split or navigate until a mouse click or other GTK focus event fires.

The fix is to find an adjacent leaf before closing and transfer focus to it:

// Find a sibling to transfer focus to before closing the pane
const next_leaf = split_tree.adjacentLeaf(root, panel_id, .next, ws.alloc);

_ = split_tree.closePane(ws.alloc, root, panel_id);
ws.removePanel(panel_id);

// Update data-model focus so split actions keep working
if (next_leaf) |l| {
    ws.focused_panel_id = l.panel_id;
    if (l.widget) |w| _ = c.gtk.gtk_widget_grab_focus(w);
} else {
    ws.focused_panel_id = null;
}

rebuildWorkspaceWidget(tm, ws);
return;