Retrieves the region that is currently being repainted.
After a call to [method@Gdk.DrawContext.begin_frame] this function will return a union of the region passed to that function and the area of the surface that the this determined needs to be repainted.
If this is not in between calls to [method@Gdk.DrawContext.begin_frame] and [ method@Gdk.DrawContext.end_frame], null will be returned.
this |
a `GdkDrawContext` |
a Cairo region |