[GTK] Implement content_insets for GTK. (#1722)

Implement content_insets for GTK.

CHANGELOG.md

@@ -34,6 +34,8 @@ You can find its changes [documented below](#070---2021-01-01).
   context-methods to implement disabled ([#1632] by [@xarvic])
 - `LifeCycle::BuildFocusChain` to update the focus-chain ([#1632] by [@xarvic])
 - `DisabledIf` widget wrapper to disable based on the state of Data and Env ([#1702] by [@xarvic])
+- GTK: added support for `content_insets` ([#1722] by [@jneem])
+
 ### Changed
 
 - Warn on unhandled Commands ([#1533] by [@Maan2003])
@@ -685,6 +687,7 @@ Last release without a changelog :(
 [#1702]: https://github.com/linebender/druid/pull/1702
 [#1713]: https://github.com/linebender/druid/pull/1713
 [#1715]: https://github.com/linebender/druid/pull/1715
+[#1722]: https://github.com/linebender/druid/pull/1722
 [#1724]: https://github.com/linebender/druid/pull/1724
 [#1730]: https://github.com/linebender/druid/pull/1730
 

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

@@ -875,9 +875,28 @@ impl WindowHandle {
         }
     }
 
+    /// The GTK implementation of content_insets differs from, e.g., the Windows one in that it
+    /// doesn't try to account for window decorations. Depending on the platform, GTK might not
+    /// even be aware of the size of the window decorations. And anyway, GTK's `Window::resize`
+    /// function [tries not to include] the window decorations, so it makes sense not to include
+    /// them here either.
+    ///
+    /// [tries not to include]: https://developer.gnome.org/gtk3/stable/GtkWidget.html#geometry-management
     pub fn content_insets(&self) -> Insets {
-        warn!("WindowHandle::content_insets unimplemented for GTK platforms.");
-        Insets::ZERO
+        if let Some(state) = self.state.upgrade() {
+            let scale = state.scale.get();
+            let (width_px, height_px) = state.window.get_size();
+            let alloc_px = state.drawing_area.get_allocation();
+            let window = Size::new(width_px as f64, height_px as f64).to_dp(scale);
+            let alloc = Rect::from_origin_size(
+                (alloc_px.x as f64, alloc_px.y as f64),
+                (alloc_px.width as f64, alloc_px.height as f64),
+            )
+            .to_dp(scale);
+            window.to_rect() - alloc
+        } else {
+            Insets::ZERO
+        }
     }
 
     pub fn set_level(&self, level: WindowLevel) {

druid-shell/src/window.rs

@@ -232,7 +232,15 @@ impl WindowHandle {
 
     /// Returns the insets of the window content from its position and size in [display points].
     ///
-    /// This is to account for any window system provided chrome, eg. title bars.
+    /// This is to account for any window system provided chrome, e.g. title bars. For example, if
+    /// you want your window to have room for contents of size `contents`, then you should call
+    /// [`WindowHandle::get_size`] with an argument of `(contents.to_rect() + insets).size()`,
+    /// where `insets` is the return value of this function.
+    ///
+    /// The details of this function are somewhat platform-dependent. For example, on Windows both
+    /// the insets and the window size include the space taken up by the title bar and window
+    /// decorations; on GTK neither the insets nor the window size include the title bar or window
+    /// decorations.
     ///
     /// [display points]: crate::Scale
     pub fn content_insets(&self) -> Insets {