Hi,

FreeCAD also has some issues with the rendering of its "start page"
where the text is missing.
And as Calibre and FreeCAD are apparently both Python applications using
QtWebEngine, it may indicate that there is something not working
correctly in QtWebEngine or in the Python bindings to QtWebEngine.

I tried printing some debug traces with:
--8<---------------cut here---------------start------------->8---
QTWEBENGINE_CHROMIUM_FLAGS="--enable-logging -v=1" ebook-viewer
QTWEBENGINE_CHROMIUM_FLAGS="--enable-logging -v=1" FreeCAD
--8<---------------cut here---------------end--------------->8---

In both cases the same error message was printed several times:
--8<---------------cut here---------------start------------->8---
[7052:1:0304/150130.882217:ERROR:paint_controller.cc(646)] PaintController::FinishCycle() completed
--8<---------------cut here---------------end--------------->8---

But so far I don't know what it means, or even if it's related to the
rendering issue...