Well I tried disabling building all examples, tests, and the debug versions of everything and my Qt5.12.9 build finished. I did a clean rebuild against Qt 5.12.9 and the bug is still there.
So it was definitely introduced inQt 5.12.8.
Of course 2 weeks back I deleted my Qt 5.12.7 build to free up disk space.
So I am now rebuilding Qt 5.12.7 from source with all patches just to verify it does NOT exhibit the bug.
If Qt 5.12.7 works, I am not sure what to do ... filing a Qt bug report about a bug that does not seem to be in Qt 5.15, seems to make little sense given how no one at Qt ever seems to listen or take bug reports seriously.
I will diff the Qt 5.12.8 qtwebengine against Qt 5.12.7 to see if anything obvious jumps out.
The problem seems related what the selection type is, as if the current selection is already a list, you can work with it correctly. It is just paragraphs it has trouble with...