Hi everyone,
As mentioned, text selection is a function of the PDF.js Core library, which we do not support.
I tried to resolve this issue by digging into the PDF.js Core, but could not come up with a good solution - the library is simply providing us invalid text location data. This can happen for a wide variety of reasons and it is impossible to come up with a single generic fix.
We will continue investigating text select issues over time, but I cannot guarantee when we will fix it or if it will ever be fixed.
Thanks,
Logan