Auto merge of #16295 - jdm:transition-safety, r=nox

Root nodes for the duration of their CSS transitions

This ensures that we can pass a node address as part of the asynchronous
transition end notification, making it safe to fire the corresponding
DOM event on the node from the script thread. Without explicitly rooting
this node when the transition starts, we risk the node being GCed before
the transition is complete.

---
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #14972
- [X] There are tests for these changes

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1 files changed, 6 insertions, 1 deletions

diff --git a/components/layout/context.rs b/components/layout/context.rs
index 214061e276f..8acbc3b254a 100644
--- a/components/layout/context.rs
+++ b/components/layout/context.rs
@@ -15,6 +15,7 @@ use net_traits::image_cache::{ImageOrMetadataAvailable, UsePlaceholder};
 use opaque_node::OpaqueNodeMethods;
 use parking_lot::RwLock;
 use script_layout_interface::{PendingImage, PendingImageState};
+use script_traits::UntrustedNodeAddress;
 use servo_url::ServoUrl;
 use std::borrow::{Borrow, BorrowMut};
 use std::cell::{RefCell, RefMut};
@@ -96,7 +97,11 @@ pub struct LayoutContext<'a> {
 
     /// A list of in-progress image loads to be shared with the script thread.
     /// A None value means that this layout was not initiated by the script thread.
-    pub pending_images: Option<Mutex<Vec<PendingImage>>>
+    pub pending_images: Option<Mutex<Vec<PendingImage>>>,
+
+    /// A list of nodes that have just initiated a CSS transition.
+    /// A None value means that this layout was not initiated by the script thread.
+    pub newly_transitioning_nodes: Option<Mutex<Vec<UntrustedNodeAddress>>>,
 }
 
 impl<'a> Drop for LayoutContext<'a> {