Florian...
Upon further investigation, the vertical scroll bar only appears on-screen when the main window is maximized if the list of tracks exceeds the height of the main window. If there are just a few tracks displayed, as when viewing the files for a single album, the scroll bar disappears when the window is maximized.
This happens with fixed monitor setups, whether wide-screen or with 4x3 displays and with dual display configurations.
This behavior seems odd to me. Of course, with just a dozen or so tracks to tag, there is no need for a vertical scroll bar. That said, if I recall correctly, I have run into situations where the scroll bar was missing when I needed it. I can't reproduce that circumstance now so perhaps I was hallucinating. 
Sorry for wandering off the thread but this is a more important issue for us. We desire a self-installing config file, so that we can easily ensure that all installations of mp3tag maintain the same configuration. I would like to be able to carry a single config file around on a memory stick and update all of our workstations once and know that they are alike. While I can do this myself easily enough, we have others that cannot, especially some volunteers working at home, a few of which are in other towns. In these cases, emailing the config file as a self-installing attachment would permit even the least knowledgeable user to update the configuration by simply double-clicking on the executable config file. As you surely know from your own experience, keeping more than two otherwise identical computers consistently alike is a daunting task over time. In our volunteer based organization, we have something like 12-15 computers that are used for tagging files.
We have an ambitious project here, creating a lossless digital music library in the FLAC file format with many thousands of albums, all tagged with an extended set of metadata to describe the tracks in great detail. In this way, the library can be searched for content in many different ways. We hope in time that artists and record labels will adopt our proposal for a larger set of FLAC tags and begin distributing and streaming digital tracks also in the FLAC file format with ample, consistent tags applied. In fact, I would like to ask you to review our proposal when it is codified.
Again, I apologize for wandering off topic. Mp3tag is a great program for us just as it is.
Dennis....aka "d2b"