Fix transparent child viewports on Windows with glow (#8423)

Fix transparent child viewports on `Windows` with `glow`

* Closes #3632
* Related #4451
* Related #5072
* Closes #7543
* Related #8116


Transparent native child viewports could become opaque on Windows when
the
selected GL config reports that it does not support transparency.

`glutin_winit::finalize_window` clears the native transparent window
attribute
in that case. However, on affected Windows GL paths, transparent native
windows
and their GL surfaces still composite correctly.

This change preserves the transparent window attribute for explicitly
transparent non-root viewports on Windows by creating those windows
directly.

Tested manually on Windows with glow:
- root transparent viewport
- deferred native child viewport
- immediate native child viewport

crates/eframe/src/native/glow_integration.rs

@@ -1268,11 +1268,30 @@ impl GlutinWindowContext {
             );
             if window_attributes.transparent()
                 && self.gl_config.supports_transparency() == Some(false)
+                && !cfg!(target_os = "windows")
             {
                 log::error!("Cannot create transparent window: the GL config does not support it");
             }
-            let window =
+
-                glutin_winit::finalize_window(event_loop, window_attributes, &self.gl_config)?;
+            let window = cfg_select! {
+                target_os = "windows" => {
+                    if viewport_id != ViewportId::ROOT && window_attributes.transparent() {
+                        // Preserve explicitly requested transparent child viewports on Windows.
+                        // Some GL paths report no transparency support although composition works.
+                        event_loop.create_window(window_attributes)?
+                    } else {
+                        glutin_winit::finalize_window(
+                            event_loop,
+                            window_attributes,
+                            &self.gl_config,
+                        )?
+                    }
+                }
+                _ => {
+                    // Keep the normal platform-specific finalization path elsewhere.
+                    glutin_winit::finalize_window(event_loop, window_attributes, &self.gl_config)?
+                }
+            };
             egui_winit::apply_viewport_builder_to_window(
                 &self.egui_ctx,
                 &window,
@@ -1497,9 +1516,17 @@ fn initialize_or_update_viewport(
             .and_then(|vp| vp.builder.icon.clone());
     }
 
+    let root_transparent = viewports
+        .get(&ViewportId::ROOT)
+        .and_then(|viewport| viewport.builder.transparent);
+
     match viewports.entry(ids.this) {
         Entry::Vacant(entry) => {
             // New viewport:
+            if ids.this != ViewportId::ROOT && builder.transparent.is_none() {
+                // Child viewports inherit the root setting unless they explicitly override it.
+                builder.transparent = root_transparent;
+            }
             log::debug!("Creating new viewport {:?} ({:?})", ids.this, builder.title);
             entry.insert(Viewport {
                 ids,