Revert removal of checking for SLINT_FULLSCREEN

env var and related docs.

Use `QWindow::setWindowStates()` to set the fullscreen state for Qt
backend.

docker/Dockerfile.torizon-demos

@@ -53,5 +53,6 @@ RUN apt-get update \
 
 COPY --from=build /slint/demos/* /usr/bin/
 
+ENV SLINT_FULLSCREEN=1
 ENV SLINT_BACKEND=winit-skia
 CMD /usr/bin/printerdemo

docs/reference/src/advanced/backend_qt.md

@@ -10,3 +10,10 @@ macOS, Windows, Linux with Wayland and X11, and direct full-screen rendering via
 The Qt backend only supports software rendering at the moment. That means it runs with any graphics driver,
 but it does not utilize GPU hardware acceleration.
 
+## Configuration Options
+
+The Qt backend reads and interprets the following environment variables:
+
+| Name               | Accepted Values | Description                                                        |
+|--------------------|-----------------|--------------------------------------------------------------------|
+| `SLINT_FULLSCREEN` | any value       | If this variable is set, every window is shown in fullscreen mode. |

docs/reference/src/advanced/backend_winit.md

@@ -17,3 +17,11 @@ The Winit backend supports different renderers. They can be explicitly selected
 | Skia Software | Software-only rendering with Skia           | `winit-skia-software`                    |
 | software      | Software-rendering, no GPU required         | `winit-software`                         |
 
+
+## Configuration Options
+
+The Winit backend reads and interprets the following environment variables:
+
+| Name               | Accepted Values | Description                                                        |
+|--------------------|-----------------|--------------------------------------------------------------------|
+| `SLINT_FULLSCREEN` | any value       | If this variable is set, every window is shown in fullscreen mode. |

examples/fullscreen_toggle/fullscreen_toggle.cpp

@@ -8,11 +8,12 @@ int main()
 {
     auto app_window = AppWindow::create();
     bool fullscreen = false;
-    app_window.on_fullscreen_toggle([fullscreen, main_window_weak = slint::ComponentWeakHandle(main_window)] {
-        auto main_window = *main_window_weak.lock();
-        fullscreen = !fullscreen;
-        main_window.window().set_fullscreen(fullscreen);
-    });
+    app_window.on_fullscreen_toggle(
+            [fullscreen, main_window_weak = slint::ComponentWeakHandle(main_window)] {
+                auto main_window = *main_window_weak.lock();
+                fullscreen = !fullscreen;
+                main_window.window().set_fullscreen(fullscreen);
+            });
 
     main_window->run();
 }

internal/backends/qt/qt_window.rs

@@ -1530,7 +1530,11 @@ impl WindowAdapter for QtWindow {
     fn set_visible(&self, visible: bool) -> Result<(), PlatformError> {
         if visible {
             let widget_ptr = self.widget_ptr();
-            cpp! {unsafe [widget_ptr as "QWidget*"] {
+            let fullscreen = std::env::var("SLINT_FULLSCREEN").is_ok();
+            cpp! {unsafe [widget_ptr as "QWidget*", fullscreen as "bool"] {
+                if (fullscreen) {
+                    widget_ptr->setWindowState(widget_ptr->windowState() | Qt::WindowFullScreen);
+                }
                 widget_ptr->show();
             }};
             let qt_platform_name = cpp! {unsafe [] -> qttypes::QString as "QString" {
@@ -1701,10 +1705,13 @@ 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(Qt::WindowFullScreen);
+                    widget_ptr->setWindowState(state | Qt::WindowFullScreen);
                 } else {
-                    widget_ptr->setWindowState(Qt::WindowNoState);
+                    widget_ptr->setWindowState(state & ~Qt::WindowFullScreen);
                 }
             }
         };

internal/backends/winit/winitwindowadapter.rs

@@ -188,6 +188,10 @@ impl WinitWindowAdapter {
         #[cfg(target_arch = "wasm32")] canvas_id: &str,
     ) -> Result<WindowBuilder, PlatformError> {
         let mut window_builder = WindowBuilder::new().with_transparent(true).with_visible(false);
+        if std::env::var("SLINT_FULLSCREEN").is_ok() {
+            window_builder =
+                window_builder.with_fullscreen(Some(winit::window::Fullscreen::Borderless(None)));
+        }
 
         window_builder = window_builder.with_title("Slint Window".to_string());