How can I get the region changed when enbale CobaltRenderDirtyRegionOnly feature?

Question

I enable CobaltRenderDirtyRegionOnly feature in cobalt. When cobalt call eglSwapBuffers(), how can I get the changed region?

I follow the https://developers.google.com/youtube/cobalt/docs/gen/cobalt/doc/performance_tuning#glyph_atlas_size to enable CobaltRenderDirtyRegionOnly

I guess it should call func eglSwapBuffersWithDamageKHR(), but it doesn't.