that is available in the internal frame history of the #GdkFrameClock.
FrameClock.getHistoryStart
long getHistoryStart()#GdkFrameClock internally keeps a history of #GdkFrameTimings objects for recent frames that can be retrieved with [gdk.frame_clock.FrameClock.getTimings]. The set of stored frames is the set from the counter values given by [gdk.frame_clock.FrameClock.getHistoryStart] and [gdk.frame_clock.FrameClock.getFrameCounter], inclusive.
Returns
the frame counter value for the oldest frame