That could happen if the backend did not provide presentation timestamps,
or if the screen was not changing other than the hardware cursor:
if (stage_cogl->last_presentation_time == 0||
stage_cogl->last_presentation_time < now - 150000)
{
stage_cogl->update_time = now;
return;
}
By setting `update_time` to `now`, master_clock_get_swap_wait_time()
returns 0:
gint64 now = g_source_get_time (master_clock->source);
if (min_update_time < now)
{
return 0;
}
else
{
gint64 delay_us = min_update_time - now;
return (delay_us + 999) / 1000;
}
However, zero is a value unsupported by the default master clock
due to:
if (swap_delay != 0)
return swap_delay;
All cases are now handled by extrapolating when the next presentation
time would be and calculating an appropriate update time to meet that.
We also need to add a check for `update_time == last_update_time`, which
is a situation that just became possible since we support old (or zero)
values of `last_presentation_time`. This avoids getting more than one
stage update per frame interval when input events arrive without
triggering a stage redraw (e.g. moving the hardware cursor).
https://gitlab.gnome.org/GNOME/mutter/merge_requests/363
35aa278194