Workaround posted. Early next week, I'll also try to write a small demo GTK program that triggers the issue, to assist with reporting upstream so the GTK folks can maybe/hopefully try to fix this.
Bug 1660223 Comment 34 Edit History
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Workaround posted. Early next week, I'll also try to write a small demo GTK program that triggers the issue, to assist with reporting upstream so the GTK folks can maybe/hopefully try to fix the root issue here, so that we don't have to work around it.