In this use case, video is not rendered to Tab, then there should not be rendering update. But rendering was updated frequently. I wonder if it might be a regression of Bug 1599656.
Bug 1605155 Comment 4 Edit History
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WebRender seems not occlude video rendering even when it is behind other items. It is expected to be dropped by using z-id by GPU
WebRender seems not occlude video rendering by cpu even when it is behind other items. It is expected to be dropped by using z-id by GPU