The error manifests itself when you copy (via button C, context menu), transfer (via button M), for some reason you need to click several times to place, then the copy disappears when you try to double click on it.
Would it be possible for you to test this using version 5.1.2? I cannot duplicate this on the latest 5.1 branch. It could be an issue with gtk2 as I am using gtk3 on my linux builds.
In my opinion, everything is much simpler. I used the git version of the program. I came to the conclusion that the overwhelming majority of errors occur for a banal reason and only in the git-version. Because in the place where errors occur, the work on one or another component has not yet been completed, it has not been brought to its logical conclusion. It is more expedient to react to bugs in the release versions (not in the git version), where all the work on the bug fixes and changes has been completed. In release 5.1.2, everything is fine with the text field. There are no such problems.
Release 5.1.2 is stable, with the exception of some artifacts from ratsnest and one crash. Users must use only latest release version, not latest git snapshot.
@Anton, we still need bug reports filed against nightly builds not just release builds. This helps us keep the nightly builds as usable as possible. That's why it is critical to include the version information in bug reports so we don't waste time trying to determine in which branch a bug exists.
I can not reproduce this in kicad 5.1.1 under ubuntu 16.04.
Could you add your version info?