[Linux] Split-pane grid — recursive binary tree

[2witstudios]

Overview

Implement the split-pane terminal grid matching macOS PaneGridController.swift. A recursive binary tree of splits, up to 6 panes.

Data Structure

enum PaneSplitNode {
    Leaf { id: String, entry: Option<TabEntry> },
    Split {
        direction: SplitDirection, // Horizontal | Vertical
        first: Box<PaneSplitNode>,
        second: Box<PaneSplitNode>,
        ratio: f64, // 0.0 to 1.0, default 0.5
    },
}

GTK Implementation

Each Split node renders as a gtk::Paned:

• Horizontal → gtk::Orientation::Vertical (top/bottom)
• Vertical → gtk::Orientation::Horizontal (left/right)
• ratio maps to Paned::set_position() based on allocated size

Each Leaf node contains either a TerminalPane (VTE) or a PlaceholderView with "New Agent" / "New Terminal" buttons.

Operations

• Split focused pane (Ctrl+D horizontal, Ctrl+Shift+D vertical): Split the leaf, create new empty leaf
• Close focused pane (Ctrl+W): Remove leaf, promote sibling
• Move focus (Ctrl+Arrow): Navigate directionally through the tree
• Max 6 leaves: Enforce limit, max 2 rows × 3 columns

Focus Management

Track focused_leaf_id. Highlight focused pane with a subtle border. Mouse click updates focus. Keyboard shortcuts navigate focus.

Hover Overlay

Each pane cell shows translucent overlay on hover with:

• Split horizontal button (⊟)
• Split vertical button (⊞)
• Close button (✕)

Use gtk::Overlay with a gtk::EventControllerMotion for hover detection.

Session Persistence

Serialize PaneSplitNode to GridLayoutNode (Codable/serde) and persist in .ppg/dashboard-sessions.json under gridLayouts[entryId]. Restore on app relaunch.

References

• macOS: PPG CLI/PPG CLI/PaneGridController.swift (1158 lines)
• Persistence: PPG CLI/PPG CLI/DashboardSession.swift (GridLayoutNode)

Acceptance Criteria

• Binary split tree renders correctly with nested gtk::Paned widgets
• Split/close/focus-move keyboard shortcuts work
• Max 6 panes enforced
• Hover overlay with split/close buttons
• Grid layout persisted and restored from dashboard-sessions.json
• Empty panes show placeholder with spawn buttons
• Drag-to-resize splits via Paned handles