feat(linux): add content_fixed overlay layer for positioned child webviews (#10420)

.changes/linux-content-fixed-overlay.md

@@ -0,0 +1,5 @@
+---
+"tao": minor
+---
+
+On Linux, add a full-window `gtk::Fixed` overlay layer to each window (alongside the default vertical `gtk::Box`), exposed via `WindowExtUnix::content_fixed()`. A runtime can build positioned child webviews into this `gtk::Fixed` so they overlay the window and honor their bounds, instead of stacking beside the default box (GTK divides a vertical box among its children, which breaks multi-webview positioning — tauri-apps/tauri#10420). The default box stays the overlay's main child, so single-webview windows are unchanged.

src/platform/unix.rs

@@ -82,6 +82,10 @@ pub trait WindowExtUnix {
   /// Returns `None` if the default vertical `gtk::Box` creation was disabled by [`WindowBuilderExtUnix::with_default_vbox`].
   fn default_vbox(&self) -> Option<&gtk::Box>;
 
+  /// Returns the full-window `gtk::Fixed` overlay layer that hosts positioned
+  /// child webviews. `None` when the default `gtk::Box` creation is disabled.
+  fn content_fixed(&self) -> Option<&gtk::Fixed>;
+
   /// Whether to show the window icon in the taskbar or not.
   fn set_skip_taskbar(&self, skip: bool) -> Result<(), ExternalError>;
 
@@ -97,6 +101,10 @@ impl WindowExtUnix for Window {
     self.window.default_vbox.as_ref()
   }
 
+  fn content_fixed(&self) -> Option<&gtk::Fixed> {
+    self.window.content_fixed.as_ref()
+  }
+
   fn set_skip_taskbar(&self, skip: bool) -> Result<(), ExternalError> {
     self.window.set_skip_taskbar(skip)
   }

src/platform_impl/linux/window.rs

@@ -49,6 +49,10 @@ pub struct Window {
   /// Gtk application window.
   pub(crate) window: gtk::ApplicationWindow,
   pub(crate) default_vbox: Option<gtk::Box>,
+  /// Full-window `gtk::Fixed` overlay layer for positioned child webviews, so a
+  /// runtime can place them over the window instead of stacking them beside the
+  /// default box. `None` when the default `gtk::Box` creation is disabled.
+  pub(crate) content_fixed: Option<gtk::Fixed>,
   /// Window requests sender
   pub(crate) window_requests_tx: glib::Sender<(WindowId, WindowRequest)>,
   scale_factor: Rc<AtomicI32>,
@@ -152,12 +156,25 @@ impl Window {
       }
     }
 
-    let default_vbox = if pl_attribs.default_vbox {
+    let (default_vbox, content_fixed) = if pl_attribs.default_vbox {
       let box_ = gtk::Box::new(gtk::Orientation::Vertical, 0);
-      window.add(&box_);
-      Some(box_)
+      // Wrap the content in a `gtk::Overlay` so positioned child webviews can be
+      // placed over the window rather than stacking beside the default box — GTK
+      // divides a vertical box among its children, which breaks multi-webview
+      // positioning (tauri-apps/tauri#10420). The default box stays the overlay's
+      // main child, so single-webview windows are unchanged; a pass-through
+      // `gtk::Fixed` overlay layer hosts positioned child webviews.
+      let overlay = gtk::Overlay::new();
+      window.add(&overlay);
+      overlay.add(&box_);
+      let fixed = gtk::Fixed::new();
+      fixed.set_halign(gtk::Align::Fill);
+      fixed.set_valign(gtk::Align::Fill);
+      overlay.add_overlay(&fixed);
+      overlay.set_overlay_pass_through(&fixed, true);
+      (Some(box_), Some(fixed))
     } else {
-      None
+      (None, None)
     };
 
     // Rest attributes
@@ -262,6 +279,7 @@ impl Window {
       window_id,
       window,
       default_vbox,
+      content_fixed,
       window_requests_tx,
       draw_tx,
       scale_factor,
@@ -405,6 +423,7 @@ impl Window {
       window_id,
       window,
       default_vbox: None,
+      content_fixed: None,
       window_requests_tx,
       draw_tx,
       scale_factor,