Only send Event::CursorExit when all mouse buttons are released

This is to allow a user to drag an element inside the program window
and for the mouse cursor to move in and out of the window during the
drag operation. This is the intuitive and expected behaviour.

3 files changed, 34 insertions, 2 deletions

diff --git a/src/lib.rs b/src/lib.rs
index acf8287..1da2e91 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -5,7 +5,7 @@ mod window_program;
 mod phosphor;
 
 pub use events::{Request, Event, Action, Axis, MouseButton, SizeBounds};
-pub(crate) use window::PhosphorWindow;
+pub(crate) use window::{PhosphorWindow, PointerState};
 pub use window_builder::WindowBuilder;
 pub use window_program::WindowProgram;
 
diff --git a/src/phosphor.rs b/src/phosphor.rs
index ab13cb0..08c71da 100644
--- a/src/phosphor.rs
+++ b/src/phosphor.rs
@@ -134,10 +134,16 @@ impl PhosphorApplication {
                 }
             }
             WindowEvent::CursorEntered { .. } => {
+                window.pointer_state = PointerState::In;
                 handle!(Event::CursorEnter);
             }
             WindowEvent::CursorLeft { .. } => {
-                handle!(Event::CursorExit);
+                if window.mouse_buttons.iter().all(|b| !b) {
+                    window.pointer_state = PointerState::Out;
+                    handle!(Event::CursorExit);
+                } else {
+                    window.pointer_state = PointerState::PendingOut;
+                }
             }
             WindowEvent::MouseWheel { delta, .. } => match delta {
                 MouseScrollDelta::LineDelta(x, y) => {
@@ -159,6 +165,20 @@ impl PhosphorApplication {
                     WinitMouseButton::Forward => handle!(Event::MouseButton { button: MouseButton::Forward, action }),
                     _ => (),
                 }
+                // Keep track of mouse button hold states, and mark cursor as out when all are released.
+                let button_index = match button {
+                    WinitMouseButton::Left    => 0,
+                    WinitMouseButton::Middle  => 1,
+                    WinitMouseButton::Right   => 2,
+                    WinitMouseButton::Back    => 3,
+                    WinitMouseButton::Forward => 4,
+                    _ => return,
+                };
+                window.mouse_buttons[button_index] = action.is_pressed();
+                if window.pointer_state == PointerState::PendingOut && window.mouse_buttons.iter().all(|b| !b) {
+                    window.pointer_state = PointerState::Out;
+                    handle!(Event::CursorExit);
+                }
             }
             WindowEvent::RedrawRequested => {
                 window.redraw();
diff --git a/src/window.rs b/src/window.rs
index 70c9c02..f031af5 100644
--- a/src/window.rs
+++ b/src/window.rs
@@ -18,11 +18,21 @@ fn to_physical_size(dimensions: Dimensions, scale: u32) -> PhysicalSize<u32> {
 }
 
 
+#[derive(Copy, Clone, Debug, PartialEq)]
+pub enum PointerState {
+    In,
+    Out,
+    PendingOut,
+}
+
+
 pub(crate) struct PhosphorWindow {
     pub winit: Rc<Window>,
     pub program: Box<dyn WindowProgram>,
     pub requests: EventQueue<Request>,
     pub pointer: Option<Position>,
+    pub pointer_state: PointerState,
+    pub mouse_buttons: [bool; 5],
     pub marked_for_destruction: bool,
     pub size_bounds: SizeBounds,  // logical dimensions
 
@@ -79,6 +89,8 @@ impl PhosphorWindow {
             program: builder.program,
             requests: EventQueue::new(),
             pointer: None,
+            pointer_state: PointerState::Out,
+            mouse_buttons: [false; 5],
             marked_for_destruction: false,
             size_bounds,