REGRESSION (r236541): [ MacOS WK1 ] Layout Test animations/stacking-context-fill-forwards.html is failing

[Truitt Savell]

The following layout test is failing on MacOS High Sierra+ WK1

animations/stacking-context-fill-forwards.html

Probable cause:

Issue seems to have been introduced within recent weeks. The diff shows a very small change in the position of several boxes. A reproduction and regression needs to be done. 

Flakiness Dashboard:

https://webkit-test-results.webkit.org/dashboards/flakiness_dashboard.html#showAllRuns=true&tests=animations%2Fstacking-context-fill-forwards.html

Diff:
https://build.webkit.org/results/Apple%20Mojave%20Release%20WK1%20(Tests)/r237352%20(459)/animations/stacking-context-fill-forwards-diffs.html

[Truitt Savell]

Regressed this change to https://trac.webkit.org/changeset/236541/webkit

I used command:
run-webkit-tests --root testbuild-236541 animations/stacking-context-fill-forwards.html --iterations 500 -f -1

this command will reproduce the flakiness consistently. 

running this with 236540 causes no failures.

[Radar WebKit Bug Importer]

<rdar://problem/45556756>

[Truitt Savell]

It seems this issue has been resolved. running the same command on a newer spade yeilds zero failures. 

comand:
run-webkit-tests --root testbuild-238794 animations/stacking-context-fill-forwards.html --iterations 5000 -f -1

marking issue resolved