I was able to reproduce the issue following the steps from comment #6 on both Win10x64 and Linux 18.04.2 LTS , using FF 70.0a1 (2019-08-07) (64-bit). I will attach a video of the issue. Also , I performed a bisection using mozregression and it looks like the problem is here : https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=26598a115edd552ce42fe28561d7b71158f06298&tochange=d05c6310d573658af4a4f2cee332bb01732fcabf
Bug 1571274 Comment 11 Edit History
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I was able to reproduce the issue following the steps from comment #3 on both Win10x64 and Linux 18.04.2 LTS , using FF 70.0a1 (2019-08-07) (64-bit). I will attach a video of the issue. Also , I performed a bisection using mozregression and it looks like the problem is here : https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=26598a115edd552ce42fe28561d7b71158f06298&tochange=d05c6310d573658af4a4f2cee332bb01732fcabf