While working on caching the display style (bug 1743968), I discovered that when MaybeQueueCacheUpdateForStyleChanges gets called for a reconstructed frame, GetFrame() returns null. This means we can't check for style changes and thus we don't send a cache update.

My guess is that when DidSetComputedStyle is called for a reconstructed frame, the frame isn't fully constructed, so nsIContent::GetPrimaryFrame doesn't know about it yet. We can fix this by calling MaybeQueueCacheUpdateForStyleChanges from PruneOrInsertSubtree instead.

We already call MaybeQueueCacheUpdateForStyleChanges from DidSetComputedStyle for reconstructed frames.
However, at that point, nsIContent::GetPrimaryFrame (and thus LocalAccessible::GetFrame) returns null, which means we're unable to check for style changes.
Instead, we now handle style changes for reconstructed frames in PruneOrInsertSubtree, at which point the frame is definitely available.

https://hg.mozilla.org/mozilla-central/rev/6f307defee29