EDIT: I could have sworn I'd documented how to build Sigil with a Qt other than the system version with the Qt5_DIR cmake option, but that must have been strictly Windows I did that for. My apologies.
I'll get that updated in the Linux docs.
The easiest way to build Sigil against a non-system Qt version is to use the cmake Qt5_DIR directive and point it to the Qt5 directory immediately under the /lib/cmake directory of the Qt5 installation you're using.
The binary installers for Qt install into /opt by default, so the bit to add to the normal cmake configure command will be something like
Code:
-DQt5_DIR=/opt/Qt5.12.3/5.12.3/gcc_64/lib/cmake/Qt5
(adjust the path for your version and architecture, of course).
After that it's just the normal make; sudo make install. The launch script will be adjusted automatically so that the new Qt version will be used instead of the system qt.
If you use PageEdit, I recommend rebuilding from the latest github. You can add the exact same -DQt5_DIR= path to its cmake configure command as the one you used for Sigil.
But I'll take a look at what's going on with the PV->CV sync code to see if something can be done to make it work with older versions of QtWebEngine. The main Sigil issue is that the Inspector won't be available with Qts older than 5.11.0. This sync problem really shouldn't be an issue in mind. If we can get you back to being able to use your system Qt5 versions we'll certainly try.
The only other thing worth mentioning is the binary installers from Qt aren't built with Proprietary Codecs enabled. So some audio/video html5 content that is supposed to work with EPUB3 probably won't work with those versions. If you don't use any audio/video in your epubs, you're good to go.