Add set_mouse_pass_through and is_foreground_window (#2402)

- `set_mouse_pass_through` sets whether the mouse passes through the
window to whatever is behind.
- `is_foreground_window` returns true if the window is the foreground
window or this is unknown, and returns false if a different window is
known to be the foreground window.

CHANGELOG.md

@@ -10,6 +10,8 @@ You can find its changes [documented below](#083---2023-02-28).
 ### Added
 
 - Type name is now included in panic error messages in `WidgetPod`. ([#2380] by [@matthewgapp])
+- `set_mouse_pass_through` sets whether the mouse passes through the window to whatever is behind. ([#2402] by [@AlexKnauth])
+- `is_foreground_window` returns true if the window is the foreground window or this is unknown, and returns false if a different window is known to be the foreground window. ([#2402] by [@AlexKnauth])
 
 ### Changed
 
@@ -790,6 +792,7 @@ Last release without a changelog :(
 [@AtomicGamer9523]: https://github.com/AtomicGamer9523
 [@Insprill]: https://github.com/Insprill
 [@matthewgapp]: https://github.com/matthewgapp
+[@AlexKnauth]: https://github.com/AlexKnauth
 
 [#599]: https://github.com/linebender/druid/pull/599
 [#611]: https://github.com/linebender/druid/pull/611
@@ -1237,6 +1240,7 @@ Last release without a changelog :(
 [#2375]: https://github.com/linebender/druid/pull/2375
 [#2378]: https://github.com/linebender/druid/pull/2378
 [#2380]: https://github.com/linebender/druid/pull/2380
+[#2402]: https://github.com/linebender/druid/pull/2402
 
 [Unreleased]: https://github.com/linebender/druid/compare/v0.8.3...master
 [0.8.3]: https://github.com/linebender/druid/compare/v0.8.2...v0.8.3

druid-shell/src/backend/gtk/window.rs

@@ -1056,6 +1056,10 @@ impl WindowHandle {
         }
     }
 
+    pub fn is_foreground_window(&self) -> bool {
+        true
+    }
+
     pub fn set_window_state(&mut self, size_state: window::WindowState) {
         use window::WindowState::{Maximized, Minimized, Restored};
         let cur_size_state = self.get_window_state();
@@ -1122,6 +1126,10 @@ impl WindowHandle {
         }
     }
 
+    pub fn set_mouse_pass_through(&self, _mouse_pass_thorugh: bool) {
+        warn!("set_mouse_pass_through unimplemented");
+    }
+
     pub fn handle_titlebar(&self, val: bool) {
         if let Some(state) = self.state.upgrade() {
             state.handle_titlebar.set(val);

druid-shell/src/backend/mac/window.rs

@@ -1335,6 +1335,13 @@ impl WindowHandle {
         }
     }
 
+    pub fn set_mouse_pass_through(&self, mouse_pass_through: bool) {
+        unsafe {
+            let window: id = msg_send![*self.nsview.load(), window];
+            window.setIgnoresMouseEvents_(mouse_pass_through as BOOL);
+        }
+    }
+
     fn set_level(&self, level: WindowLevel) {
         unsafe {
             let level = levels::as_raw_window_level(level);
@@ -1355,6 +1362,14 @@ impl WindowHandle {
         }
     }
 
+    pub fn is_foreground_window(&self) -> bool {
+        unsafe {
+            let application: id = msg_send![class![NSRunningApplication], currentApplication];
+            let is_active: BOOL = msg_send![application, isActive];
+            is_active != NO
+        }
+    }
+
     pub fn get_window_state(&self) -> WindowState {
         unsafe {
             let window: id = msg_send![*self.nsview.load(), window];

druid-shell/src/backend/wayland/window.rs

@@ -109,6 +109,10 @@ impl WindowHandle {
         tracing::warn!("set_always_on_top is unimplemented on wayland");
     }
 
+    pub fn set_mouse_pass_through(&self, _mouse_pass_thorugh: bool) {
+        tracing::warn!("set_mouse_pass_through unimplemented");
+    }
+
     pub fn set_input_region(&self, region: Option<Region>) {
         self.inner.surface.set_input_region(region);
     }
@@ -130,6 +134,10 @@ impl WindowHandle {
         self.inner.surface.get_size()
     }
 
+    pub fn is_foreground_window(&self) -> bool {
+        true
+    }
+
     pub fn set_window_state(&mut self, _current_state: window::WindowState) {
         tracing::warn!("set_window_state is unimplemented on wayland");
     }

druid-shell/src/backend/web/window.rs

@@ -510,6 +510,10 @@ impl WindowHandle {
         warn!("WindowHandle::set_always_on_top unimplemented for web");
     }
 
+    pub fn set_mouse_pass_through(&self, _mouse_pass_thorugh: bool) {
+        warn!("WindowHandle::set_mouse_pass_through unimplemented for web");
+    }
+
     pub fn get_position(&self) -> Point {
         warn!("WindowHandle::get_position unimplemented for web.");
         Point::new(0.0, 0.0)
@@ -524,6 +528,10 @@ impl WindowHandle {
         Size::new(0.0, 0.0)
     }
 
+    pub fn is_foreground_window(&self) -> bool {
+        true
+    }
+
     pub fn content_insets(&self) -> Insets {
         warn!("WindowHandle::content_insets unimplemented for web.");
         Insets::ZERO

druid-shell/src/backend/windows/window.rs

@@ -89,6 +89,7 @@ pub(crate) struct WindowBuilder {
     position: Option<Point>,
     level: Option<WindowLevel>,
     always_on_top: bool,
+    mouse_pass_through: bool,
     state: window::WindowState,
 }
 
@@ -156,6 +157,7 @@ enum DeferredOp {
     ReleaseMouseCapture,
     SetRegion(Option<Region>),
     SetAlwaysOnTop(bool),
+    SetMousePassThrough(bool),
 }
 
 #[derive(Clone, Debug)]
@@ -232,6 +234,7 @@ struct WindowState {
     is_focusable: bool,
     window_level: WindowLevel,
     is_always_on_top: Cell<bool>,
+    is_mouse_pass_through: Cell<bool>,
 }
 
 impl std::fmt::Debug for WindowState {
@@ -464,6 +467,38 @@ fn set_ex_style(hwnd: HWND, always_on_top: bool) {
     }
 }
 
+fn set_mouse_pass_through(hwnd: HWND, mouse_pass_through: bool) {
+    unsafe {
+        let mut style = GetWindowLongPtrW(hwnd, GWL_EXSTYLE) as u32;
+        if style == 0 {
+            warn!(
+                "failed to get window ex style: {}",
+                Error::Hr(HRESULT_FROM_WIN32(GetLastError()))
+            );
+            return;
+        }
+
+        if !mouse_pass_through {
+            // Not removing WS_EX_LAYERED because it may still be needed if Opacity != 1.
+            style &= !WS_EX_TRANSPARENT;
+        } else if (style & (WS_EX_LAYERED | WS_EX_TRANSPARENT))
+            != (WS_EX_LAYERED | WS_EX_TRANSPARENT)
+        {
+            // We have to add WS_EX_LAYERED, because WS_EX_TRANSPARENT won't work otherwise.
+            style |= WS_EX_LAYERED | WS_EX_TRANSPARENT;
+        } else {
+            // nothing to do
+            return;
+        }
+        if SetWindowLongPtrW(hwnd, GWL_EXSTYLE, style as _) == 0 {
+            warn!(
+                "failed to set the window ex style: {}",
+                Error::Hr(HRESULT_FROM_WIN32(GetLastError()))
+            );
+        }
+    }
+}
+
 impl WndState {
     fn rebuild_render_target(&mut self, d2d: &D2DFactory, scale: Scale) -> Result<(), Error> {
         unsafe {
@@ -647,6 +682,10 @@ impl MyWndProc {
                     self.with_window_state(|s| s.is_always_on_top.set(always_on_top));
                     set_ex_style(hwnd, always_on_top);
                 }
+                DeferredOp::SetMousePassThrough(mouse_pass_through) => {
+                    self.with_window_state(|s| s.is_mouse_pass_through.set(mouse_pass_through));
+                    set_mouse_pass_through(hwnd, mouse_pass_through);
+                }
                 DeferredOp::SetWindowState(val) => {
                     let show = if self.handle.borrow().is_focusable() {
                         match val {
@@ -1490,6 +1529,7 @@ impl WindowBuilder {
             position: None,
             level: None,
             always_on_top: false,
+            mouse_pass_through: false,
             state: window::WindowState::Restored,
         }
     }
@@ -1640,6 +1680,7 @@ impl WindowBuilder {
                 is_focusable: focusable,
                 window_level,
                 is_always_on_top: Cell::new(self.always_on_top),
+                is_mouse_pass_through: Cell::new(self.mouse_pass_through),
             };
             let win = Rc::new(window);
             let handle = WindowHandle {
@@ -2244,6 +2285,14 @@ impl WindowHandle {
         Size::new(0.0, 0.0)
     }
 
+    pub fn is_foreground_window(&self) -> bool {
+        let Some(w) = self.state.upgrade() else {
+            return true;
+        };
+        let hwnd = w.hwnd.get();
+        unsafe { GetForegroundWindow() == hwnd }
+    }
+
     pub fn set_input_region(&self, area: Option<Region>) {
         self.defer(DeferredOp::SetRegion(area));
     }
@@ -2252,6 +2301,10 @@ impl WindowHandle {
         self.defer(DeferredOp::SetAlwaysOnTop(always_on_top));
     }
 
+    pub fn set_mouse_pass_through(&self, mouse_pass_through: bool) {
+        self.defer(DeferredOp::SetMousePassThrough(mouse_pass_through));
+    }
+
     pub fn resizable(&self, resizable: bool) {
         self.defer(DeferredOp::SetResizable(resizable));
     }

druid-shell/src/backend/x11/window.rs

@@ -1675,6 +1675,10 @@ impl WindowHandle {
         }
     }
 
+    pub fn set_mouse_pass_through(&self, _mouse_pass_thorugh: bool) {
+        warn!("set_mouse_pass_through unimplemented");
+    }
+
     pub fn set_input_region(&self, region: Option<Region>) {
         if let Some(w) = self.window.upgrade() {
             w.set_input_region(region);
@@ -1714,6 +1718,10 @@ impl WindowHandle {
         }
     }
 
+    pub fn is_foreground_window(&self) -> bool {
+        true
+    }
+
     pub fn set_window_state(&self, _state: window::WindowState) {
         warn!("WindowHandle::set_window_state is currently unimplemented for X11 backend.");
     }

druid-shell/src/window.rs

@@ -225,6 +225,11 @@ impl WindowHandle {
         self.0.set_always_on_top(always_on_top);
     }
 
+    /// Sets whether the mouse passes through the window to whatever is behind.
+    pub fn set_mouse_pass_through(&self, mouse_pass_through: bool) {
+        self.0.set_mouse_pass_through(mouse_pass_through);
+    }
+
     /// Sets where in the window the user can interact with the program.
     ///
     /// This enables irregularly shaped windows. For example, you can make it simply
@@ -244,6 +249,12 @@ impl WindowHandle {
         self.0.set_input_region(region)
     }
 
+    /// Returns true if the window is the foreground window or this is unknown.
+    /// Returns false if a different window is known to be the foreground window.
+    pub fn is_foreground_window(&self) -> bool {
+        self.0.is_foreground_window()
+    }
+
     /// Returns the position of the top left corner of the window.
     ///
     /// The position is returned in [display points], measured relative to the parent window if

druid/examples/input_region.rs

@@ -26,6 +26,8 @@ struct AppState {
     limit_input_region: bool,
     show_titlebar: bool,
     always_on_top: bool,
+    mouse_pass_through_while_not_in_focus: bool,
+    mouse_pass_through: bool,
 }
 
 struct InputRegionExampleWidget {
@@ -38,7 +40,7 @@ impl InputRegionExampleWidget {
         let info_label = Label::new(INFO_TEXT)
             .with_line_break_mode(LineBreaking::WordWrap)
             .padding(20.0)
-            .background(Color::rgba(0.2, 0.2, 0.2, 1.0));
+            .background(Color::rgba(0.2, 0.2, 0.2, 0.5));
         let toggle_input_region = Button::new("Toggle Input Region")
             .on_click(|ctx, data: &mut bool, _: &Env| {
                 *data = !*data;
@@ -61,10 +63,20 @@ impl InputRegionExampleWidget {
                 ctx.window().set_always_on_top(*data);
             })
             .lens(AppState::always_on_top);
+        let toggle_mouse_pass_through_while_not_in_focus = Button::new("Toggle Mouse Pass Through")
+            .on_click(|_, data: &mut bool, _: &Env| {
+                *data = !*data;
+                tracing::debug!(
+                    "Setting mouse pass through while not in focus to: {}",
+                    *data
+                );
+            })
+            .lens(AppState::mouse_pass_through_while_not_in_focus);
         let controls_flex = Flex::row()
             .with_child(toggle_input_region)
             .with_child(toggle_titlebar)
-            .with_child(toggle_always_on_top);
+            .with_child(toggle_always_on_top)
+            .with_child(toggle_mouse_pass_through_while_not_in_focus);
         Self {
             info_label: WidgetPod::new(info_label),
             controls: WidgetPod::new(controls_flex),
@@ -82,6 +94,13 @@ impl Widget<AppState> for InputRegionExampleWidget {
     ) {
         self.info_label.event(ctx, event, data, env);
         self.controls.event(ctx, event, data, env);
+        let mouse_pass_through =
+            data.mouse_pass_through_while_not_in_focus && !ctx.window().is_foreground_window();
+        if mouse_pass_through != data.mouse_pass_through {
+            data.mouse_pass_through = mouse_pass_through;
+            tracing::debug!("Setting mouse pass through to: {}", mouse_pass_through);
+            ctx.window().set_mouse_pass_through(mouse_pass_through);
+        }
     }
 
     fn lifecycle(
@@ -196,6 +215,8 @@ fn main() {
         limit_input_region: true,
         always_on_top: false,
         show_titlebar: false,
+        mouse_pass_through_while_not_in_focus: false,
+        mouse_pass_through: false,
     };
 
     AppLauncher::with_window(main_window)