Auto merge of #17866 - hiikezoe:restyle-damage, r=emilio

Preserve restyle damage if the element was not restyled during thrott…

…led animation flush.

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https://bugzilla.mozilla.org/show_bug.cgi?id=1381420

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors

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ports/geckolib/glue.rs

@@ -2779,8 +2779,13 @@ pub extern "C" fn Servo_TakeChangeHint(element: RawGeckoElementBorrowed,
 
             let damage = data.restyle.damage;
             if restyle_behavior == structs::TraversalRestyleBehavior::ForThrottledAnimationFlush {
-                debug_assert!(data.restyle.is_restyle() || damage.is_empty(),
-                              "Restyle damage should be empty if the element was not restyled");
+                if !*was_restyled {
+                    // Don't touch elements if the element was not restyled
+                    // in throttled animation flush.
+                    debug!("Skip post traversal for throttled animation flush {:?} restyle={:?}",
+                           element, data.restyle);
+                    return nsChangeHint(0);
+                }
                 // In the case where we call this function for post traversal for
                 // flusing throttled animations (i.e. without normal restyle
                 // traversal), we need to preserve restyle hints for normal restyle