Implement top-left hot corner to toggle the Overview

Compared to third-party implementations such as waycorner:

- It works during interactive window move (no surfaces receive pointer
  focus in this case, so this cannot work through layer-shell).
- It works during drag-and-drop.
- It disables itself over fullscreen windows.
- It does not prevent direct scanout.

niri-config/src/lib.rs

@@ -1210,6 +1210,8 @@ pub struct Gestures {
     pub dnd_edge_view_scroll: DndEdgeViewScroll,
     #[knuffel(child, default)]
     pub dnd_edge_workspace_switch: DndEdgeWorkspaceSwitch,
+    #[knuffel(child, default)]
+    pub hot_corners: HotCorners,
 }
 
 #[derive(knuffel::Decode, Debug, Clone, Copy, PartialEq)]
@@ -1252,6 +1254,12 @@ impl Default for DndEdgeWorkspaceSwitch {
     }
 }
 
+#[derive(knuffel::Decode, Debug, Default, Clone, Copy, PartialEq)]
+pub struct HotCorners {
+    #[knuffel(child)]
+    pub off: bool,
+}
+
 #[derive(knuffel::Decode, Debug, Clone, Copy, PartialEq)]
 pub struct Overview {
     #[knuffel(child, unwrap(argument), default = Self::default().zoom)]
@@ -4587,6 +4595,9 @@ mod tests {
                         1500.0,
                     ),
                 },
+                hot_corners: HotCorners {
+                    off: false,
+                },
             },
             overview: Overview {
                 zoom: FloatOrInt(

src/input/mod.rs

@@ -29,7 +29,7 @@ use smithay::input::touch::{
 };
 use smithay::input::SeatHandler;
 use smithay::output::Output;
-use smithay::utils::{Logical, Point, Rectangle, Transform, SERIAL_COUNTER};
+use smithay::utils::{Logical, Point, Rectangle, Size, Transform, SERIAL_COUNTER};
 use smithay::wayland::keyboard_shortcuts_inhibit::KeyboardShortcutsInhibitor;
 use smithay::wayland::pointer_constraints::{with_pointer_constraint, PointerConstraint};
 use smithay::wayland::selection::data_device::DnDGrab;
@@ -1995,6 +1995,10 @@ impl State {
     }
 
     fn on_pointer_motion<I: InputBackend>(&mut self, event: I::PointerMotionEvent) {
+        let was_inside_hot_corner = self.niri.pointer_inside_hot_corner;
+        // Any of the early returns here mean that the pointer is not inside the hot corner.
+        self.niri.pointer_inside_hot_corner = false;
+
         // We need an output to be able to move the pointer.
         if self.niri.global_space.outputs().next().is_none() {
             return;
@@ -2171,6 +2175,22 @@ impl State {
 
         pointer.frame(self);
 
+        // contents_under() will return no surface when the hot corner should trigger.
+        let hot_corners = self.niri.config.borrow().gestures.hot_corners;
+        if !hot_corners.off
+            && pointer.current_focus().is_none()
+            && !self.niri.screenshot_ui.is_open()
+        {
+            let hot_corner = Rectangle::from_size(Size::from((1., 1.)));
+            if let Some((_, pos_within_output)) = self.niri.output_under(pos) {
+                let inside_hot_corner = hot_corner.contains(pos_within_output);
+                if inside_hot_corner && !was_inside_hot_corner {
+                    self.niri.layout.toggle_overview();
+                }
+                self.niri.pointer_inside_hot_corner = inside_hot_corner;
+            }
+        }
+
         // Activate a new confinement if necessary.
         self.niri.maybe_activate_pointer_constraint();
 
@@ -2195,6 +2215,10 @@ impl State {
         &mut self,
         event: I::PointerMotionAbsoluteEvent,
     ) {
+        let was_inside_hot_corner = self.niri.pointer_inside_hot_corner;
+        // Any of the early returns here mean that the pointer is not inside the hot corner.
+        self.niri.pointer_inside_hot_corner = false;
+
         let Some(pos) = self.compute_absolute_location(&event, None).or_else(|| {
             self.global_bounding_rectangle().map(|output_geo| {
                 event.position_transformed(output_geo.size) + output_geo.loc.to_f64()
@@ -2240,6 +2264,22 @@ impl State {
 
         pointer.frame(self);
 
+        // contents_under() will return no surface when the hot corner should trigger.
+        let hot_corners = self.niri.config.borrow().gestures.hot_corners;
+        if !hot_corners.off
+            && pointer.current_focus().is_none()
+            && !self.niri.screenshot_ui.is_open()
+        {
+            let hot_corner = Rectangle::from_size(Size::from((1., 1.)));
+            if let Some((_, pos_within_output)) = self.niri.output_under(pos) {
+                let inside_hot_corner = hot_corner.contains(pos_within_output);
+                if inside_hot_corner && !was_inside_hot_corner {
+                    self.niri.layout.toggle_overview();
+                }
+                self.niri.pointer_inside_hot_corner = inside_hot_corner;
+            }
+        }
+
         self.niri.maybe_activate_pointer_constraint();
 
         // We moved the pointer, show it.

src/niri.rs

@@ -347,6 +347,7 @@ pub struct Niri {
     /// Used for limiting the notify to once per iteration, so that it's not spammed with high
     /// resolution mice.
     pub notified_activity_this_iteration: bool,
+    pub pointer_inside_hot_corner: bool,
     pub tablet_cursor_location: Option<Point<f64, Logical>>,
     pub gesture_swipe_3f_cumulative: Option<(f64, f64)>,
     pub overview_scroll_swipe_gesture: ScrollSwipeGesture,
@@ -2427,6 +2428,7 @@ impl Niri {
             pointer_inactivity_timer: None,
             pointer_inactivity_timer_got_reset: false,
             notified_activity_this_iteration: false,
+            pointer_inside_hot_corner: false,
             tablet_cursor_location: None,
             gesture_swipe_3f_cumulative: None,
             overview_scroll_swipe_gesture: ScrollSwipeGesture::new(),
@@ -2920,6 +2922,11 @@ impl Niri {
             return false;
         }
 
+        let hot_corner = Rectangle::from_size(Size::from((1., 1.)));
+        if hot_corner.contains(pos_within_output) {
+            return true;
+        }
+
         if layer_popup_under(Layer::Top) || layer_toplevel_under(Layer::Top) {
             return true;
         }
@@ -3175,6 +3182,11 @@ impl Niri {
                 .or_else(|| layer_toplevel_under(Layer::Bottom))
                 .or_else(|| layer_toplevel_under(Layer::Background));
         } else {
+            let hot_corner = Rectangle::from_size(Size::from((1., 1.)));
+            if hot_corner.contains(pos_within_output) {
+                return rv;
+            }
+
             under = under
                 .or_else(|| layer_popup_under(Layer::Top))
                 .or_else(|| layer_toplevel_under(Layer::Top));