Custom Window Resize for Borderless Windows

Overview

This document describes the custom window resize functionality implemented for Ferrite's borderless window mode. Since the application uses with_decorations(false) to provide a custom title bar, native OS resize handles are not available. This module provides custom edge/corner detection and resize initiation.

Architecture

Module Location

• Source: src/ui/window.rs
• Integration: src/app.rs

Key Components

WindowResizeState

Tracks the current resize operation state:

pub struct WindowResizeState {
    current_direction: Option<ResizeDirection>,
    is_resizing: bool,
}

• current_direction: The detected resize direction based on mouse position (None if not near an edge)
• is_resizing: Whether a resize operation is currently active (mouse held down)

handle_window_resize()

The main entry point called at the start of each frame:

1. Early exit if maximized: Resize is disabled when window is maximized
2. Get window rect and pointer position: Required for edge detection
3. Continue active resize: If a resize is in progress, maintain state until mouse is released
4. Detect resize direction: Check if pointer is near any edge/corner
5. Set cursor icon: Change cursor to indicate resize capability
6. Initiate resize: On mouse press, send ViewportCommand::BeginResize(direction) to egui

Resize Zones

The implementation defines two key constants:

• RESIZE_BORDER_WIDTH (5px): The width of edge resize zones
• CORNER_GRAB_SIZE (10px): The size of corner grab areas (larger for easier targeting)

Edge Detection Logic

The detect_resize_direction() function:

1. Checks proximity to each edge (left, right, top, bottom)
2. Prioritizes corners over edges (corners are checked first)
3. Returns the appropriate ResizeDirection variant

Supported directions:

• North, South, East, West (edges)
• NorthEast, NorthWest, SouthEast, SouthWest (corners)

Cursor Icons

Each direction maps to a corresponding cursor icon:

Direction	Cursor Icon
North	ResizeNorth (↑)
South	ResizeSouth (↓)
East	ResizeEast (→)
West	ResizeWest (←)
NorthEast	ResizeNorthEast (↗)
NorthWest	ResizeNorthWest (↖)
SouthEast	ResizeSouthEast (↘)
SouthWest	ResizeSouthWest (↙)

Integration with Title Bar

The title bar drag-to-move functionality is updated to defer to resize handling:

// Handle drag to move window (but not if we're in a resize zone)
let is_in_resize = self.window_resize_state.current_direction().is_some()
    || self.window_resize_state.is_resizing();
if drag_response.dragged() && !is_in_resize {
    ctx.send_viewport_cmd(egui::ViewportCommand::StartDrag);
}

This prevents conflicts between window move and resize operations when the mouse is near edges.

Frame Order

The resize handling must be called early in the frame, before any UI rendering:

fn update(&mut self, ctx: &egui::Context, _frame: &mut eframe::Frame) {
    // Handle window resize for borderless window (must be early, before UI)
    handle_window_resize(ctx, &mut self.window_resize_state);
    
    // ... rest of UI rendering
    
    // Block edge widgets from stealing resize clicks (end of frame, Foreground order)
    consume_clicks_in_resize_zones(ctx, &self.window_resize_state);
    self.window_resize_state.apply_cursor(ctx);
}

This ensures:

1. Resize state is updated before UI checks it
2. Cursor icon changes are applied immediately
3. Resize commands are sent before any UI interaction
4. Foreground click guards (wider than the 5px cursor zone) win hit-testing over edge UI such as the side-panel toggle strip and Help button

Click blocking vs. moving buttons

Do not fix resize/click conflicts by moving buttons away from edges. Use:

• WindowResizeState::blocks_widget_clicks() — true while the resize cursor is shown, during resize, or for one frame after release over a grab zone
• consume_clicks_in_resize_zones() — paints invisible Foreground hit targets using get_click_block_zone_rect() (24px on east/west vs. 5px detection)

Platform Considerations

• Windows: Fully supported via winit/eframe integration
• macOS: Should work similarly (winit handles platform specifics)
• Linux: Supported on X11 and Wayland (winit handles platform specifics)

Note: Platform-specific behavior may vary slightly depending on the window manager.

Testing Checklist

• Cursor changes to resize icons when hovering edges
• Cursor changes to diagonal icons when hovering corners
• All 8 resize directions work correctly
• Minimum window size is enforced (400x300)
• Resize is disabled when window is maximized
• Title bar drag-to-move works in center area
• Title bar does not initiate move when dragging from edges
• Double-click on title bar still toggles maximize
• Smooth 60fps performance during resize

API Reference

Public Functions

/// Handle window resize for borderless windows.
/// Returns `true` if a resize operation was initiated.
pub fn handle_window_resize(ctx: &egui::Context, state: &mut WindowResizeState) -> bool

/// Check if a pointer position is within the resize border zone.
pub fn is_in_resize_zone(window_rect: Rect, pointer_pos: Pos2) -> bool

/// Get the resize zone rectangle for a given edge.
pub fn get_resize_zone_rect(window_rect: Rect, edge: ResizeDirection) -> Rect

WindowResizeState Methods

/// Create a new resize state.
pub fn new() -> Self

/// Check if currently resizing.
pub fn is_resizing(&self) -> bool

/// Get current resize direction.
pub fn current_direction(&self) -> Option<ResizeDirection>

Dependencies

• eframe::egui - UI framework with viewport commands
• egui::ResizeDirection - Direction enum for resize operations
• egui::ViewportCommand::BeginResize - Command to initiate native resize