Added handle_titlebar support on GTK backend (linebender#2230)

Author: Steve-xmh

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

@@ -174,6 +174,7 @@ pub(crate) struct WindowState {
     scale: Cell<Scale>,
     area: Cell<ScaledArea>,
     is_transparent: Cell<bool>,
+    handle_titlebar: Cell<bool>,
     /// Used to determine whether to honor close requests from the system: we inhibit them unless
     /// this is true, and this gets set to true when our client requests a close.
     closing: Cell<bool>,
@@ -353,6 +354,7 @@ impl WindowBuilder {
             scale: Cell::new(scale),
             area: Cell::new(area),
             is_transparent: Cell::new(transparent),
+            handle_titlebar: Cell::new(false),
             closing: Cell::new(false),
             drawing_area,
             surface: RefCell::new(None),
@@ -580,6 +582,10 @@ impl WindowBuilder {
                                 },
                             );
                         }
+                        if button.is_left() && state.handle_titlebar.replace(false) {
+                            let (root_x, root_y) = event.root();
+                            state.window.begin_move_drag(event.button() as i32, root_x as i32, root_y as i32, event.time());
+                        }
                     }
                 });
             }
@@ -604,6 +610,9 @@ impl WindowBuilder {
                                 wheel_delta: Vec2::ZERO
                             },
                         );
+                        if button.is_left() {
+                            state.handle_titlebar.set(false);
+                        }
                     }
                 });
             }
@@ -1092,8 +1101,10 @@ impl WindowHandle {
         Restored
     }
 
-    pub fn handle_titlebar(&self, _val: bool) {
-        warn!("WindowHandle::handle_titlebar is currently unimplemented for gtk.");
+    pub fn handle_titlebar(&self, val: bool) {
+        if let Some(state) = self.state.upgrade() {
+            state.handle_titlebar.set(val);
+        }
     }
 
     /// Close the window.

druid-shell/src/window.rs

@@ -206,7 +206,7 @@ impl WindowHandle {
     /// because this refers to the current location of the mouse, you should probably call this
     /// function in response to every relevant [`WinHandler::mouse_move`].
     ///
-    /// This is currently only implemented on Windows.
+    /// This is currently only implemented on Windows and GTK.
     pub fn handle_titlebar(&self, val: bool) {
         self.0.handle_titlebar(val);
     }

druid/examples/transparency.rs

@@ -18,16 +18,33 @@
 // On Windows platform, don't show a console when opening the app.
 #![windows_subsystem = "windows"]
 
-use druid::kurbo::Circle;
 use druid::widget::prelude::*;
 use druid::widget::{Flex, Label, Painter, TextBox, WidgetExt};
+use druid::{kurbo::Circle, widget::Controller};
 use druid::{AppLauncher, Color, Lens, Rect, WindowDesc};
 
 #[derive(Clone, Data, Lens)]
 struct HelloState {
     name: String,
 }
 
+struct DragController;
+
+impl<T, W: Widget<T>> Controller<T, W> for DragController {
+    fn event(
+        &mut self,
+        _child: &mut W,
+        ctx: &mut EventCtx,
+        event: &Event,
+        _data: &mut T,
+        _env: &Env,
+    ) {
+        if let Event::MouseMove(_) = event {
+            ctx.window().handle_titlebar(true);
+        }
+    }
+}
+
 pub fn main() {
     let window = WindowDesc::new(build_root_widget())
         .show_titlebar(false)
@@ -81,7 +98,7 @@ fn build_root_widget() -> impl Widget<HelloState> {
     .with_text_size(32.0);
 
     Flex::column()
-        .with_flex_child(circle_and_rects.expand(), 10.0)
+        .with_flex_child(circle_and_rects.expand().controller(DragController), 10.0)
         .with_spacer(4.0)
         .with_child(textbox)
         .with_spacer(4.0)