56.1.0 crashes with Speed Start add-on when Stylo is enabled

[JustOff]

STR:

• Install Speed Start
• Open new tab
• Use right mouse button's menu to create a link to some site
• Restart WF
• Open new tab

WF 56.0.x works just fine and no crushes are observed.

Known workaround is to set layout.css.servo.enabled -> false. Tag #511.

[grahamperrin]

Not crashing with Stylo with 56.1.0_2 on FreeBSD-CURRENT.

Compatibility reports: https://addons.mozilla.org/compatibility/reporter/SStart%40Off.JustOff

I added links to:

• https://addons.mozilla.org/en-GB/firefox/addon/speed-start/
• 56.1.0 crashes with Speed Start add-on when Stylo is enabled #512
• https://github.com/MrAlex94/Waterfox/issues?q=is%3Aissue+is%3Aopen+sort%3Aupdated-desc

[grahamperrin]

If I:

• enable multi-process Waterfox (ignore the reported incompatibility with e10s) and
• add a link to a page that's known to make a tab crash with Stylo

– then Waterfox will crash at the attempt to create the thumbnail.

If I disable Stylo before restoring the crashed session, then I can create the thumbnail, re-enable Stylo and open a new tab without a crash. Waterfox will crash if I attempt to refresh the thumbnail.

[jbeich]

@grahamperrin, FreeBSD package is unaffected because it backported mozilla/gecko-dev@6905267b2878.