feat(linux): support adding to gtk::Fixed (#1128)

* feat(linux): support adding to `gtk::Fixed`

* fix doctests

* again

.changes/gtk-fixed.md

@@ -0,0 +1,5 @@
+---
+"wry": "patch"
+---
+
+On Linux, apply passed webview bounds when using `WebView::new_gtk` or `WebViewBuilder::new_gtk` with `gtk::Fixed` widget. This allows to create multiple webviews inside `gtk::Fixed` in the same window.

examples/gtk_multiwebview.rs

@@ -0,0 +1,134 @@
+// Copyright 2020-2023 Tauri Programme within The Commons Conservancy
+// SPDX-License-Identifier: Apache-2.0
+// SPDX-License-Identifier: MIT
+
+use tao::{
+  event::{Event, WindowEvent},
+  event_loop::{ControlFlow, EventLoop},
+  window::WindowBuilder,
+};
+use wry::{Rect, WebViewBuilder};
+
+fn main() -> wry::Result<()> {
+  let event_loop = EventLoop::new();
+  let window = WindowBuilder::new().build(&event_loop).unwrap();
+
+  #[cfg(not(any(
+    target_os = "windows",
+    target_os = "macos",
+    target_os = "ios",
+    target_os = "android"
+  )))]
+  let fixed = {
+    use gtk::prelude::*;
+    use tao::platform::unix::WindowExtUnix;
+
+    let fixed = gtk::Fixed::new();
+    let vbox = window.default_vbox().unwrap();
+    vbox.pack_start(&fixed, true, true, 0);
+    fixed.show_all();
+    fixed
+  };
+
+  let create_webview_builder = || {
+    #[cfg(any(
+      target_os = "windows",
+      target_os = "macos",
+      target_os = "ios",
+      target_os = "android"
+    ))]
+    return WebViewBuilder::new_as_child(&window);
+
+    #[cfg(not(any(
+      target_os = "windows",
+      target_os = "macos",
+      target_os = "ios",
+      target_os = "android"
+    )))]
+    {
+      use wry::WebViewBuilderExtUnix;
+      WebViewBuilder::new_gtk(&fixed)
+    }
+  };
+
+  let size = window.inner_size().to_logical::<u32>(window.scale_factor());
+
+  let webview = create_webview_builder()
+    .with_bounds(Rect {
+      x: 0,
+      y: 0,
+      width: size.width / 2,
+      height: size.height / 2,
+    })
+    .with_url("https://tauri.app")?
+    .build()?;
+  let webview2 = create_webview_builder()
+    .with_bounds(Rect {
+      x: (size.width / 2) as i32,
+      y: 0,
+      width: size.width / 2,
+      height: size.height / 2,
+    })
+    .with_url("https://github.com/tauri-apps/wry")?
+    .build()?;
+  let webview3 = create_webview_builder()
+    .with_bounds(Rect {
+      x: 0,
+      y: (size.height / 2) as i32,
+      width: size.width / 2,
+      height: size.height / 2,
+    })
+    .with_url("https://twitter.com/TauriApps")?
+    .build()?;
+  let webview4 = create_webview_builder()
+    .with_bounds(Rect {
+      x: (size.width / 2) as i32,
+      y: (size.height / 2) as i32,
+      width: size.width / 2,
+      height: size.height / 2,
+    })
+    .with_url("https://google.com")?
+    .build()?;
+
+  event_loop.run(move |event, _, control_flow| {
+    *control_flow = ControlFlow::Wait;
+
+    match event {
+      Event::WindowEvent {
+        event: WindowEvent::Resized(size),
+        ..
+      } => {
+        let size = size.to_logical::<u32>(window.scale_factor());
+        webview.set_bounds(Rect {
+          x: 0,
+          y: 0,
+          width: size.width / 2,
+          height: size.height / 2,
+        });
+        webview2.set_bounds(Rect {
+          x: (size.width / 2) as i32,
+          y: 0,
+          width: size.width / 2,
+          height: size.height / 2,
+        });
+        webview3.set_bounds(Rect {
+          x: 0,
+          y: (size.height / 2) as i32,
+          width: size.width / 2,
+          height: size.height / 2,
+        });
+        webview4.set_bounds(Rect {
+          x: (size.width / 2) as i32,
+          y: (size.height / 2) as i32,
+          width: size.width / 2,
+          height: size.height / 2,
+        });
+      }
+      Event::WindowEvent {
+        event: WindowEvent::CloseRequested,
+        ..
+      } => *control_flow = ControlFlow::Exit,
+      _ => {}
+    }
+  });
+}