For native WR compositors instead of WR compositors, this is already reported as individual dirty rects via start_compositing. Take a look at how SwCompositor uses it. I am guessing set_buffer_damage_region was only added as a hack to support legacy draw compositors, but we should really start moving away from those. Sotaro's work on a GL compositor for android will probably allow you to just use that native compositor instead of needing RenderCompositorEGL at all. Likewise, that GL compositor could then choose to make use of the information about dirty rects passed in via start_compositing.
Bug 1640712 Comment 3 Edit History
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For native WR compositors instead of draw compositors, this is already reported as individual dirty rects via start_compositing. Take a look at how SwCompositor uses it. I am guessing set_buffer_damage_region was only added as a hack to support legacy draw compositors, but we should really start moving away from those. Sotaro's work on a GL compositor for android will probably allow you to just use that native compositor instead of needing RenderCompositorEGL at all. Likewise, that GL compositor could then choose to make use of the information about dirty rects passed in via start_compositing.