getHistoryStart
Returns the frame counter for the oldest frame available in history.
GdkFrameClock internally keeps a history of GdkFrameTimings objects for recent frames that can be retrieved with method@Gdk.FrameClock.get_timings. The set of stored frames is the set from the counter values given by method@Gdk.FrameClock.get_history_start and method@Gdk.FrameClock.get_frame_counter, inclusive.
Return
the frame counter value for the oldest frame that is available in the internal frame history of the GdkFrameClock