Added set_fullscreen API to WindowAdapter.

internal/backends/qt/qt_window.rs

@@ -1533,7 +1533,7 @@ impl WindowAdapter for QtWindow {
             let fullscreen = std::env::var("SLINT_FULLSCREEN").is_ok();
             cpp! {unsafe [widget_ptr as "QWidget*", fullscreen as "bool"] {
                 if (fullscreen) {
-                    widget_ptr->setWindowState(Qt::WindowFullScreen);
+                    widget_ptr->setWindowState(widget_ptr->windowState() | Qt::WindowFullScreen);
                 }
                 widget_ptr->show();
             }};
@@ -1702,6 +1702,21 @@ impl WindowAdapter for QtWindow {
         }};
     }
 
+    fn set_fullscreen(&self, fullscreen: bool) {
+        let widget_ptr = self.widget_ptr();
+        cpp! {unsafe [widget_ptr as "QWidget*", fullscreen as "bool"] {
+                // Depending on the request, we either set or clear the fullscreen bits.
+                // See also: https://doc.qt.io/qt-6/qt.html#WindowState-enum
+                const auto state = widget_ptr->windowState();
+                if (fullscreen) {
+                    widget_ptr->setWindowState(state | Qt::WindowFullScreen);
+                } else {
+                    widget_ptr->setWindowState(state & ~Qt::WindowFullScreen);
+                }
+            }
+        };
+    }
+
     fn internal(&self, _: i_slint_core::InternalToken) -> Option<&dyn WindowAdapterInternal> {
         Some(self)
     }

internal/backends/winit/winitwindowadapter.rs

@@ -561,6 +561,14 @@ impl WindowAdapter for WinitWindowAdapter {
         });
     }
 
+    fn set_fullscreen(&self, fullscreen: bool) {
+        if fullscreen {
+            self.winit_window.set_fullscreen(Some(winit::window::Fullscreen::Borderless(None)));
+        } else {
+            self.winit_window.set_fullscreen(None);
+        }
+    }
+
     fn internal(&self, _: corelib::InternalToken) -> Option<&dyn WindowAdapterInternal> {
         Some(self)
     }

internal/core/api.rs

@@ -446,6 +446,11 @@ impl Window {
         crate::window::WindowAdapter::set_size(&*self.0.window_adapter(), size);
     }
 
+    /// Set or unset the window to display fullscreen.
+    pub fn set_fullscreen(&self, fullscreen: bool) {
+        self.0.window_adapter().set_fullscreen(fullscreen);
+    }
+
     /// Dispatch a window event to the scene.
     ///
     /// Use this when you're implementing your own backend and want to forward user input events.

internal/core/window.rs

@@ -132,6 +132,9 @@ pub trait WindowAdapter {
     /// be called again.
     fn update_window_properties(&self, _properties: WindowProperties<'_>) {}
 
+    /// Set to display the window as fullscreen.
+    fn set_fullscreen(&self, _fullscreen: bool) {}
+
     #[doc(hidden)]
     fn internal(&self, _: crate::InternalToken) -> Option<&dyn WindowAdapterInternal> {
         None

tools/lsp/main.rs

@@ -239,10 +239,6 @@ fn main() {
     if !args.backend.is_empty() {
         std::env::set_var("SLINT_BACKEND", &args.backend);
     }
-    if args.fullscreen {
-        // TODO: Have an API to set the Window fullscreen #3283
-        std::env::set_var("SLINT_FULLSCREEN", "1");
-    }
 
     #[cfg(feature = "preview-engine")]
     {

tools/lsp/preview/native.rs

@@ -150,7 +150,7 @@ pub fn open_ui(sender: &ServerNotifier) {
 }
 
 fn open_ui_impl(preview_state: &mut PreviewState) {
-    let (default_style, show_preview_ui) = {
+    let (default_style, show_preview_ui, fullscreen) = {
         let cache = super::CONTENT_CACHE.get_or_init(Default::default).lock().unwrap();
         let style = cache.config.style.clone();
         let style = if style.is_empty() {
@@ -163,12 +163,14 @@ fn open_ui_impl(preview_state: &mut PreviewState) {
             .hide_ui
             .or_else(|| CLI_ARGS.with(|args| args.get().map(|a| a.no_toolbar.clone())))
             .unwrap_or(false);
-        (style, !hide_ui)
+        let fullscreen = CLI_ARGS.with(|args| args.get().map(|a| a.fullscreen).unwrap_or_default());
+        (style, !hide_ui, fullscreen)
     };
 
     // TODO: Handle Error!
     let ui = preview_state.ui.get_or_insert_with(|| super::ui::create_ui(default_style).unwrap());
     ui.set_show_preview_ui(show_preview_ui);
+    ui.window().set_fullscreen(fullscreen);
     ui.window().on_close_requested(|| {
         let mut cache = super::CONTENT_CACHE.get_or_init(Default::default).lock().unwrap();
         cache.ui_is_visible = false;