Allow users to create viewports larger than monitor on Windows & macOS (#4337)

Added clamp_size_to_monitor_size field on ViewportBuilder, which means
whether clamp the window's size to monitor's size. (default to `true`)

* Closes https://github.com/emilk/egui/issues/3389

### simple example

```rust
pub struct MyApp {}

impl MyApp {
    pub fn new() -> MyApp {
        MyApp {}
    }
}

impl eframe::App for MyApp {
    fn update(&mut self, ctx: &Context, frame: &mut eframe::Frame) {
        egui::CentralPanel::default()
            .frame(Frame::none().fill(Color32::DARK_GRAY))
            .show(ctx, |ui| {
                if ctx.input(|i| i.key_pressed(Key::Escape)) {
                    ctx.send_viewport_cmd(ViewportCommand::Close);
                }
            });
    }
}

pub fn main() {
    let option = eframe::NativeOptions {
        viewport: ViewportBuilder::default()
            .with_position([10.0, 10.0])
            .with_inner_size([3000.0, 2000.0])
            .with_clamp_size_to_monitor_size(false),
        ..Default::default()
    };

    eframe::run_native(
        "a large window app",
        option,
        Box::new(|ctx| Box::new(MyApp::new())),
    ).unwrap();
}
```

It works on my windows (with 3 monitors), but I don't have a test
environment for macos

---------

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

crates/eframe/src/native/epi_integration.rs

@@ -20,14 +20,23 @@ pub fn viewport_builder<E>(
 
     let mut viewport_builder = native_options.viewport.clone();
 
+    let clamp_size_to_monitor_size = viewport_builder.clamp_size_to_monitor_size.unwrap_or(
+        // On some Linux systems, a window size larger than the monitor causes crashes
+        cfg!(target_os = "linux"),
+    );
+
     // Always use the default window size / position on iOS. Trying to restore the previous position
     // causes the window to be shown too small.
     #[cfg(not(target_os = "ios"))]
     let inner_size_points = if let Some(mut window_settings) = window_settings {
         // Restore pos/size from previous session
 
-        window_settings
-            .clamp_size_to_sane_values(largest_monitor_point_size(egui_zoom_factor, event_loop));
+        if clamp_size_to_monitor_size {
+            window_settings.clamp_size_to_sane_values(largest_monitor_point_size(
+                egui_zoom_factor,
+                event_loop,
+            ));
+        }
         window_settings.clamp_position_to_monitors(egui_zoom_factor, event_loop);
 
         viewport_builder = window_settings.initialize_viewport_builder(viewport_builder);
@@ -37,11 +46,13 @@ pub fn viewport_builder<E>(
             viewport_builder = viewport_builder.with_position(pos);
         }
 
+        if clamp_size_to_monitor_size {
             if let Some(initial_window_size) = viewport_builder.inner_size {
                 let initial_window_size = initial_window_size
                     .at_most(largest_monitor_point_size(egui_zoom_factor, event_loop));
                 viewport_builder = viewport_builder.with_inner_size(initial_window_size);
             }
+        }
 
         viewport_builder.inner_size
     };

crates/egui-winit/src/lib.rs

@@ -1598,6 +1598,7 @@ pub fn create_winit_window_builder<T>(
         window_type: _window_type,
 
         mouse_passthrough: _, // handled in `apply_viewport_builder_to_window`
+        clamp_size_to_monitor_size: _, // Handled in `viewport_builder` in `epi_integration.rs`
     } = viewport_builder;
 
     let mut window_builder = winit::window::WindowBuilder::new()

crates/egui/src/viewport.rs

@@ -275,6 +275,11 @@ pub struct ViewportBuilder {
     pub min_inner_size: Option<Vec2>,
     pub max_inner_size: Option<Vec2>,
 
+    /// Whether clamp the window's size to monitor's size. The default is `true` on linux, otherwise it is `false`.
+    ///
+    /// Note: On some Linux systems, a window size larger than the monitor causes crashes
+    pub clamp_size_to_monitor_size: Option<bool>,
+
     pub fullscreen: Option<bool>,
     pub maximized: Option<bool>,
     pub resizable: Option<bool>,
@@ -493,6 +498,15 @@ impl ViewportBuilder {
         self
     }
 
+    /// Sets whether clamp the window's size to monitor's size. The default is `true` on linux, otherwise it is `false`.
+    ///
+    /// Note: On some Linux systems, a window size larger than the monitor causes crashes
+    #[inline]
+    pub fn with_clamp_size_to_monitor_size(mut self, value: bool) -> Self {
+        self.clamp_size_to_monitor_size = Some(value);
+        self
+    }
+
     /// Does not work on X11.
     #[inline]
     pub fn with_close_button(mut self, value: bool) -> Self {
@@ -606,6 +620,7 @@ impl ViewportBuilder {
             inner_size: new_inner_size,
             min_inner_size: new_min_inner_size,
             max_inner_size: new_max_inner_size,
+            clamp_size_to_monitor_size: new_clamp_size_to_monitor_size,
             fullscreen: new_fullscreen,
             maximized: new_maximized,
             resizable: new_resizable,
@@ -740,6 +755,13 @@ impl ViewportBuilder {
 
         let mut recreate_window = false;
 
+        if new_clamp_size_to_monitor_size.is_some()
+            && self.clamp_size_to_monitor_size != new_clamp_size_to_monitor_size
+        {
+            self.clamp_size_to_monitor_size = new_clamp_size_to_monitor_size;
+            recreate_window = true;
+        }
+
         if new_active.is_some() && self.active != new_active {
             self.active = new_active;
             recreate_window = true;