We may end up with window-added emitted multiple times on the
same window, which results on:
1) Extra clones being created, all taking the same size and
stacking.
2) JS exceptions because handle each clone actor being destroyed
one by one, but all clones already have a NULL source when the
first destroy handler is called, so we step on null objects
inside _computeBoundingBox().
Keep accounting of those windows in order to avoid multiple
additions, which fixes both issues.
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