Consider the example app in #148742 . This uses a backdrop blur to achieve an iOS like overlay effect. Despite covering only a small region of the screen, for Impeller we will process and restore the entire screen to implement the backdrop.

There are other bdf filter approaches that are more optimial if only a small region of the screen is covered (or if there are multiple non-overlapping bdfs). One would be to replay the onscreen commands into an offscreen buffer with tighter bounds.

For a CI test case, the existing backdrop filter benchmark (which regressed with Impeller) is a reasonable test case for this and other optimizations.

https://flutter-flutter-perf.skia.org/e/?queries=device_type%3DPixel_7_Pro%26sub_result%3D90th_percentile_frame_rasterizer_time_millis%26sub_result%3D99th_percentile_frame_rasterizer_time_millis%26sub_result%3Daverage_frame_rasterizer_time_millis%26sub_result%3Dworst_frame_rasterizer_time_millis%26test%3Dbackdrop_filter_perf__timeline_summary