Spell checking suggestions is not working in since bug 1646648.

STR:
Write "test aaa".
aaa gets the red underline, but no suggestions in the context menu, like there used do

Bah, this works in the one and only test for it, because that test doesn't run in the compose window.

The suggestions broke because document.popupRangeParent/Offset provide the wrong information for some reason.
Fortunately the context menu actor already provides all the information we need, it just needs to be hooked up.

Pushed by mkmelin@iki.fi:
https://hg.mozilla.org/comm-central/rev/db4c36ca9386
Fix spelling suggestions in compose context menu. r=mkmelin

Comment on attachment 9199115 [details]
Bug 1688551 - Fix spelling suggestions in compose context menu. r?mkmelin

[Approval Request Comment]
Regression caused by (bug #): tb-fission
User impact if declined: spell check suggestions in compose missing
Testing completed (on c-c, etc.): landed ~12h ago
Risk to taking this patch (and alternatives if risky): I've discovered a few other spell check issues caused by this patch but I think it's worth it

Comment on attachment 9199115 [details]
Bug 1688551 - Fix spelling suggestions in compose context menu. r?mkmelin

[Triage Comment]
Approved for beta

Thunderbird 86.0b1:
https://hg.mozilla.org/releases/comm-beta/rev/36f436488a89
https://hg.mozilla.org/releases/comm-beta/rev/6d9b0cb8b47b

On windows 10 I can confirm this is fixed in build2 of 86.0b1.