This reverts commit b56b23007664525e13f7f7ec50a9a6adff0bdfd9.
which somehow made us miss repaints on uvar notifications.
The commit was a workaround for a polling bug which was later properly
fixed by 7c5b8b855 ("Use the uvar notifier pipe timestamp to avoid
excessive polling"), so it's no longer necessary.
Add a system test. If I had a better understanding of the bug I could
probably write a better test.
Fixes#8088
