The first failure that I see this on the dashboard was 09/03/20 at r266538 on Catalina debug wk2. It has had a frequency of approximately every 4 days, but on 09/16/20 increased in frequency to daily occurrences.
I can reproduce this on r266932, but unable to reproduce on r266931.
Commit r266932 has to do with MediaToolboxSoftLink.cpp and MediaToolboxSoftLink.h, which could have introduced the flakiness. The change was introduced here https://trac.webkit.org/changeset/266932/webkit.
run-webkit-tests --iterations 2000 --exit-after-n-failures 1 --exit-after-n-crashes-or-timeouts 1 --debug-rwt-logging --no-retry --force --no-build -f fast/scrolling/latching/latched-scroll-into-nonfast-region.html
Regressions: Unexpected text-only failures (1)
fast/scrolling/latching/latched-scroll-into-nonfast-region.html [ Failure ]
@@ -4,7 +4,7 @@
Scrolling the main frame with latching
-PASS wheelEventHandlerCount is 0
+FAIL wheelEventHandlerCount should be 0. Was 3.
PASS successfullyParsed is true
This issue is also affecting Mojave.
I have marked this test as failing also on Mojave while this issue is investigated.
Looking at test history (both webkit.org and internal), I think that this was just landed flaky. Frequency mysteriously changes over time, but some bots saw multiple failures a day from the start.