On X11, allow building window with parent

src/platform/x11.rs

@@ -1,6 +1,7 @@
 use std::os::raw;
 use std::{ptr, sync::Arc};
 
+use crate::window::WindowId;
 use crate::{
     event_loop::{EventLoopBuilder, EventLoopWindowTarget},
     monitor::MonitorHandle,
@@ -171,6 +172,8 @@ pub trait WindowBuilderExtX11 {
     fn with_x11_visual<T>(self, visual_infos: *const T) -> Self;
 
     fn with_x11_screen(self, screen_id: i32) -> Self;
+    /// Build window with parent window.
+    fn with_parent(self, parent_id: WindowId) -> Self;
 
     /// Build window with the given `general` and `instance` names.
     ///
@@ -227,6 +230,12 @@ impl WindowBuilderExtX11 for WindowBuilder {
         self
     }
 
+    #[inline]
+    fn with_parent(mut self, parent_id: WindowId) -> Self {
+        self.platform_specific.parent_id = Some(parent_id.0);
+        self
+    }
+
     #[inline]
     fn with_override_redirect(mut self, override_redirect: bool) -> Self {
         self.platform_specific.override_redirect = override_redirect;

src/platform_impl/linux/mod.rs

@@ -95,6 +95,8 @@ pub struct PlatformSpecificWindowBuilderAttributes {
     #[cfg(feature = "x11")]
     pub screen_id: Option<i32>,
     #[cfg(feature = "x11")]
+    pub parent_id: Option<WindowId>,
+    #[cfg(feature = "x11")]
     pub base_size: Option<Size>,
     #[cfg(feature = "x11")]
     pub override_redirect: bool,
@@ -115,6 +117,8 @@ impl Default for PlatformSpecificWindowBuilderAttributes {
             #[cfg(feature = "x11")]
             screen_id: None,
             #[cfg(feature = "x11")]
+            parent_id: None,
+            #[cfg(feature = "x11")]
             base_size: None,
             #[cfg(feature = "x11")]
             override_redirect: false,

src/platform_impl/linux/x11/window.rs

@@ -119,7 +119,12 @@ impl UnownedWindow {
         pl_attribs: PlatformSpecificWindowBuilderAttributes,
     ) -> Result<UnownedWindow, RootOsError> {
         let xconn = &event_loop.xconn;
-        let root = event_loop.root;
+        let root = if let Some(id) = pl_attribs.parent_id {
+            // WindowId is XID under the hood which doesn't exceed u32, so this conversion is lossless
+            u64::from(id) as _
+        } else {
+            event_loop.root
+        };
 
         let mut monitors = xconn.available_monitors();
         let guessed_monitor = if monitors.is_empty() {