x11: fix WindowAttributesExtX11::with_x11_screen()

Based on #3973, which should be merged first.

There's an API to programmatically specify X11 screen id (override what is determined from the `DISPLAY` env variable), but it doesn't work.

Seeting up X Server with 2 screens and calling `DISPLAY=:0 X11_SCREEN_ID=1 cargo run --example window` should be equivalent to calling `DISPLAY=:0.1 cargo run --example window`

The latter works (and places the window on the correct screen), but the former yields

`failed to create initial window: Os(OsError { line: 620, file: "src/platform_impl/linux/x11/window.rs", error: X11Error(X11Error { error_kind: Match, error_code: 8, sequence: 219, bad_value: 1319, minor_opcode: 0, major_opcode: 1, extension_name: None, request_name: Some("CreateWindow") }) })`

_Here `1319` is the root window id for screen 0, which doesn't match the screen 1 that we request._

The problem is that we need to factor in the screen id when determining the parent (root) window when not explicitly set. This patch does that.

---

Also: Extend the window example with X11_{SCREEN,VISUAL}_ID env variables

examples/window.rs

@@ -31,6 +31,8 @@ use winit::platform::startup_notify::{
 };
 #[cfg(web_platform)]
 use winit::platform::web::{ActiveEventLoopExtWeb, CustomCursorExtWeb, WindowAttributesExtWeb};
+#[cfg(x11_platform)]
+use winit::platform::x11::WindowAttributesExtX11;
 use winit::window::{
     Cursor, CursorGrabMode, CustomCursor, CustomCursorSource, Fullscreen, Icon, ResizeDirection,
     Theme, Window, WindowAttributes, WindowId,
@@ -149,6 +151,28 @@ impl Application {
             window_attributes = window_attributes.with_activation_token(token);
         }
 
+        #[cfg(x11_platform)]
+        match std::env::var("X11_VISUAL_ID") {
+            Ok(visual_id_str) => {
+                info!("Using X11 visual id {visual_id_str}");
+                let visual_id = visual_id_str.parse()?;
+                window_attributes = window_attributes.with_x11_visual(visual_id);
+            },
+            Err(_) => info!("Set the X11_VISUAL_ID env variable to request specific X11 visual"),
+        }
+
+        #[cfg(x11_platform)]
+        match std::env::var("X11_SCREEN_ID") {
+            Ok(screen_id_str) => {
+                info!("Placing the window on X11 screen {screen_id_str}");
+                let screen_id = screen_id_str.parse()?;
+                window_attributes = window_attributes.with_x11_screen(screen_id);
+            },
+            Err(_) => info!(
+                "Set the X11_SCREEN_ID env variable to place the window on non-default screen"
+            ),
+        }
+
         #[cfg(macos_platform)]
         if let Some(tab_id) = _tab_id {
             window_attributes = window_attributes.with_tabbing_identifier(&tab_id);

src/changelog/unreleased.md

@@ -70,6 +70,8 @@ changelog entry.
 - Add `PointerKind`, `PointerSource`, `ButtonSource`, `FingerId`, `primary` and `position` to all
   pointer events as part of the pointer event overhaul.
 - Add `DeviceId::into_raw()` and `from_raw()`.
+- On X11, the `window` example now understands the `X11_VISUAL_ID` and `X11_SCREEN_ID` env
+  variables to test the respective modifiers of window creation.
 
 ### Changed
 
@@ -196,3 +198,4 @@ changelog entry.
 - On X11, key events forward to IME anyway, even when it's disabled.
 - On Windows, make `ControlFlow::WaitUntil` work more precisely using `CREATE_WAITABLE_TIMER_HIGH_RESOLUTION`.
 - On X11, creating windows on screen that is not the first one (e.g. `DISPLAY=:0.1`) works again.
+- On X11, creating windows while passing `with_x11_screen(non_default_screen)` works again.

src/platform_impl/linux/x11/window.rs

@@ -439,11 +439,25 @@ impl UnownedWindow {
     ) -> Result<UnownedWindow, RequestError> {
         let xconn = &event_loop.xconn;
         let atoms = xconn.atoms();
+
+        let screen_id = match window_attrs.platform_specific.x11.screen_id {
+            Some(id) => id,
+            None => xconn.default_screen_index() as c_int,
+        };
+
+        let screen = {
+            let screen_id_usize = usize::try_from(screen_id)
+                .map_err(|_| NotSupportedError::new("screen id must be non-negative"))?;
+            xconn.xcb_connection().setup().roots.get(screen_id_usize).ok_or(
+                NotSupportedError::new("requested screen id not present in server's response"),
+            )?
+        };
+
         let root = match window_attrs.parent_window.as_ref().map(|handle| handle.0) {
             Some(rwh_06::RawWindowHandle::Xlib(handle)) => handle.window as xproto::Window,
             Some(rwh_06::RawWindowHandle::Xcb(handle)) => handle.window.get(),
             Some(raw) => unreachable!("Invalid raw window handle {raw:?} on X11"),
-            None => event_loop.root,
+            None => screen.root,
         };
 
         let mut monitors = leap!(xconn.available_monitors());
@@ -499,19 +513,6 @@ impl UnownedWindow {
             dimensions
         };
 
-        let screen_id = match window_attrs.platform_specific.x11.screen_id {
-            Some(id) => id,
-            None => xconn.default_screen_index() as c_int,
-        };
-
-        let screen = {
-            let screen_id_usize = usize::try_from(screen_id)
-                .map_err(|_| NotSupportedError::new("screen id must be non-negative"))?;
-            xconn.xcb_connection().setup().roots.get(screen_id_usize).ok_or(
-                NotSupportedError::new("requested screen id not present in server's response"),
-            )?
-        };
-
         // An iterator over the visuals matching screen id combined with their depths.
         let mut all_visuals = screen
             .allowed_depths