fix(linux): position gtk::Fixed child webviews via move_ (honor set_bounds)

[ds1]

What

On Linux, WebView::set_bounds positions a gtk::Fixed-parented webview with a bare size_allocate. That allocation is transient — GtkFixed re-allocates each child to its stored put-coordinate on the next size-allocate, so the requested position is dropped on the first relayout (the webview snaps back to where it was first placed). The size is also not retained as a request.

This positions the child via its parent gtk::Fixed's move_ and set_size_request (which persist across layout passes), keeping the immediate size_allocate:

if self.is_in_fixed_parent {
  if let Some(fixed) = self.webview.parent().and_then(|p| p.downcast::<gtk::Fixed>().ok()) {
    fixed.move_(&self.webview, x, y);
  }
  self.webview.set_size_request(width, height);
  self.webview.size_allocate(&gtk::Allocation::new(x, y, width, height));
}

Why

This is the documented X11 + Wayland multi-webview path — WebViewBuilderExtUnix::new_gtk / build_gtk over a gtk::Fixed (see the gtk_multiwebview example). Today, set_bounds calls after the initial layout don't hold: positioned webviews drift back to their original spot on resize/relayout.

Context: this is the wry-side half of tauri-apps/tauri#10420 (multi-webview child positioning broken on Linux/GTK). The other half is Tauri packing child webviews into a gtk::Fixed rather than the window's vertical gtk::Box (proposed in that thread). With a gtk::Fixed parent + this change, set_bounds from app code positions children correctly and the position survives relayouts.

Testing

Applied on top of wry 0.55.1 and run in a Tauri v2 app on WSLg (Ubuntu 22.04, WebKitGTK 2.50.4), with content webviews hosted in a gtk::Fixed: webviews now hold their bounds across tab-switch, multi-tab, drawer-resize, and window-resize. Without move_, the position was dropped after the first relayout (reproduced and verified).

Notes

Related (not in this PR): on this path WebView::bounds() doesn't report the child's position (returns (0, 0)), so a read-modify-write of bounds (e.g. set_size after set_position) can clobber the position. Happy to follow up separately.

A .changes entry is included (wry: patch).