It seems to be a ubuntu problem with any qt/kde app that uses QtWebEngine. Here is a related report that fails inside that same libgallium version as well.
https://www.mail-archive.com/kde-bug...sg1164460.html
And the same issue was reported for falkon and the workaround was:
[alex@alexarch ~]$ export QT_QUICK_BACKEND=software
[alex@alexarch ~]$ falkon
So try Sigil after exporting that exact same env var.
See:
https://bbs.archlinux.org/viewtopic.php?id=312838
And from that post:
Quote:
- forcing NVIDIA gpu on system makes Falkon work
- And, indeed, installing vulkan-radeon fixes it when using dual mode.
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So you could try that as well.
Given that last post above, before downgrading libgallium I would try adding the vulkan-radeon package