eframe: add function to set, query and toggle fullscreen mode (#1866)

Closes https://github.com/emilk/egui/pull/674

Adds `NativeOptions:fullscreen`, `Frame::set_fullscreen` and `WindowInfo::fullscreen`.

eframe/src/epi.rs

@@ -180,6 +180,11 @@ pub struct NativeOptions {
     /// If false it will be difficult to move and resize the app.
     pub decorated: bool,
 
+    /// Start in (borderless) fullscreen?
+    ///
+    /// Default: `false`.
+    pub fullscreen: bool,
+
     /// On Windows: enable drag and drop support. Drag and drop can
     /// not be disabled on other platforms.
     ///
@@ -270,6 +275,7 @@ impl Default for NativeOptions {
             always_on_top: false,
             maximized: false,
             decorated: true,
+            fullscreen: false,
             drag_and_drop_support: true,
             icon_data: None,
             initial_window_pos: None,
@@ -547,11 +553,17 @@ impl Frame {
     }
 
     /// Set whether to show window decorations (i.e. a frame around you app).
+    ///
     /// If false it will be difficult to move and resize the app.
     pub fn set_decorations(&mut self, decorated: bool) {
         self.output.decorated = Some(decorated);
     }
 
+    /// Turn borderless fullscreen on/off (native only).
+    pub fn set_fullscreen(&mut self, fullscreen: bool) {
+        self.output.fullscreen = Some(fullscreen);
+    }
+
     /// set the position of the outer window
     pub fn set_window_pos(&mut self, pos: egui::Pos2) {
         self.output.window_pos = Some(pos);
@@ -591,6 +603,9 @@ pub struct WindowInfo {
     /// Unit: egui points (logical pixels).
     pub position: egui::Pos2,
 
+    /// Are we in fullscreen mode?
+    pub fullscreen: bool,
+
     /// Window inner size in egui points (logical pixels).
     pub size: egui::Vec2,
 }
@@ -740,6 +755,9 @@ pub mod backend {
         /// Set to some bool to change window decorations.
         pub decorated: Option<bool>,
 
+        /// Set to some bool to change window fullscreen.
+        pub fullscreen: Option<bool>,
+
         /// Set to true to drag window while primary mouse button is down.
         pub drag_window: bool,
 

eframe/src/native/epi_integration.rs

@@ -21,6 +21,7 @@ pub fn read_window_info(
                 .to_logical::<f32>(pixels_per_point.into());
             Some(WindowInfo {
                 position: egui::Pos2 { x: pos.x, y: pos.y },
+                fullscreen: window.fullscreen().is_some(),
                 size: egui::Vec2 {
                     x: size.width,
                     y: size.height,
@@ -39,6 +40,7 @@ pub fn window_builder(
         always_on_top,
         maximized,
         decorated,
+        fullscreen,
         drag_and_drop_support,
         icon_data,
         initial_window_pos,
@@ -54,8 +56,9 @@ pub fn window_builder(
 
     let mut window_builder = winit::window::WindowBuilder::new()
         .with_always_on_top(*always_on_top)
-        .with_maximized(*maximized)
         .with_decorations(*decorated)
+        .with_fullscreen(fullscreen.then(|| winit::window::Fullscreen::Borderless(None)))
+        .with_maximized(*maximized)
         .with_resizable(*resizable)
         .with_transparent(*transparent)
         .with_window_icon(window_icon);
@@ -118,6 +121,7 @@ pub fn handle_app_output(
         window_size,
         window_title,
         decorated,
+        fullscreen,
         drag_window,
         window_pos,
         visible,
@@ -137,6 +141,10 @@ pub fn handle_app_output(
         );
     }
 
+    if let Some(fullscreen) = fullscreen {
+        window.set_fullscreen(fullscreen.then(|| winit::window::Fullscreen::Borderless(None)));
+    }
+
     if let Some(window_title) = window_title {
         window.set_title(&window_title);
     }

eframe/src/web/backend.rs

@@ -298,6 +298,7 @@ impl AppRunner {
                 window_size: _,  // Can't resize a web page
                 window_title: _, // TODO(emilk): change title of window
                 decorated: _,    // Can't toggle decorations
+                fullscreen: _,   // TODO(emilk): fullscreen web window
                 drag_window: _,  // Can't be dragged
                 window_pos: _,   // Can't set position of a web page
                 visible: _,      // Can't hide a web page