If I test release builds, the bug doesn't reproduce. However, looking at about:config, gfx.direct3d11.use-double-buffering is set to false by default, even on 68 and 69. If I manually flip to true and restart, it reproduces. Not sure 1) why it's false by default and 2) why Nightly still reproduces with the pref set either way. But at least this doesn't appear to be actively affecting Release users from what I can see.
Bug 1586130 Comment 12 Edit History
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If I test release builds, the bug doesn't reproduce. However, looking at about:config, gfx.direct3d11.use-double-buffering was restricted back to Nightly-only via bug 1555956. If I manually flip the pref to true and restart, it reproduces. Not sure why Nightly still reproduces with the pref set either way, though. But at least this doesn't appear to be actively affecting Release users from what I can see.