Support mouse scroll in X11.

CHANGELOG.md

@@ -99,6 +99,7 @@ This means that druid no longer requires cairo on macOS and uses Core Graphics i
 - Routing `LifeCycle::FocusChanged` to descendant widgets. ([#925] by [@yrns])
 - Built-in open and save menu items now show the correct label and submit the right commands. ([#930] by [@finnerale])
 - Wheel events now properly update hot state. ([#951] by [@xStrom])
+- X11: Support mouse scrolling. ([#961] by [@jneem])
 
 ### Visual
 
@@ -200,6 +201,7 @@ This means that druid no longer requires cairo on macOS and uses Core Graphics i
 [#953]: https://github.com/xi-editor/druid/pull/953
 [#954]: https://github.com/xi-editor/druid/pull/954
 [#959]: https://github.com/xi-editor/druid/pull/959
+[#961]: https://github.com/xi-editor/druid/pull/961
 
 ## [0.5.0] - 2020-04-01
 

druid-shell/src/platform/x11/application.rs

@@ -183,7 +183,6 @@ impl Application {
         self.screen_num
     }
 
-    // TODO(x11/events): handle mouse scroll events
     #[allow(clippy::cognitive_complexity)]
     pub fn run(self, _handler: Option<Box<dyn AppHandler>>) {
         loop {
@@ -226,7 +225,16 @@ impl Application {
                         let window_id = button_press.event();
                         match self.window(window_id) {
                             Ok(w) => {
-                                if let Err(err) = w.handle_button_press(button_press) {
+                                // X doesn't have dedicated scroll events: it uses mouse buttons instead.
+                                // Buttons 4/5 are vertical; 6/7 are horizontal.
+                                if button_press.detail() >= 4 && button_press.detail() <= 7 {
+                                    if let Err(err) = w.handle_wheel(button_press) {
+                                        log::error!(
+                                            "BUTTON_PRESS - failed to handle wheel: {}",
+                                            err
+                                        );
+                                    }
+                                } else if let Err(err) = w.handle_button_press(button_press) {
                                     log::error!("BUTTON_PRESS - failed to handle: {}", err);
                                 }
                             }
@@ -238,7 +246,10 @@ impl Application {
                         let window_id = button_release.event();
                         match self.window(window_id) {
                             Ok(w) => {
-                                if let Err(err) = w.handle_button_release(button_release) {
+                                if button_release.detail() >= 4 && button_release.detail() <= 7 {
+                                    // This is the release event corresponding to a mouse wheel.
+                                    // Ignore it: we already handled the press event.
+                                } else if let Err(err) = w.handle_button_release(button_release) {
                                     log::error!("BUTTON_RELEASE - failed to handle: {}", err);
                                 }
                             }

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

@@ -602,6 +602,42 @@ impl Window {
         }
     }
 
+    pub fn handle_wheel(&self, event: &xcb::ButtonPressEvent) -> Result<(), Error> {
+        let button = event.detail();
+        // The scroll delta of 120.0 was stolen from the GTK code, which apparently came from the
+        // windows docs.
+        let delta = match button {
+            4 => (0.0, -120.0),
+            5 => (0.0, 120.0),
+            6 => (-120.0, 0.0),
+            7 => (120.0, 0.0),
+            _ => {
+                log::error!("unexpected mouse wheel button: {}", button);
+                (0.0, 0.0)
+            }
+        };
+        let mouse_event = MouseEvent {
+            pos: Point::new(event.event_x() as f64, event.event_y() as f64),
+            buttons: mouse_buttons(event.state()),
+            mods: key_mods(event.state()),
+            count: 0,
+            focus: false,
+            button: MouseButton::None,
+            wheel_delta: delta.into(),
+        };
+
+        match self.handler.try_borrow_mut() {
+            Ok(mut handler) => {
+                handler.wheel(&mouse_event);
+                Ok(())
+            }
+            Err(err) => Err(Error::BorrowError(format!(
+                "Window::handle_wheel handle: {}",
+                err
+            ))),
+        }
+    }
+
     pub fn handle_motion_notify(
         &self,
         motion_notify: &xcb::MotionNotifyEvent,