Thanks for the steps and sample files. I'm still not seeing this on my own Ubuntu 12.04 test system. We do have some differences introduced in our toolbar behavior in BC4, so we may have a specific issue that's difficult to reproduce. I did try:
1) Launch BC4
2) Session menu -> New Session -> Folder Compare
3a) Session menu -> Left click and hold, then release
3b) Sessions toolbar button -> left click and hold, then release.

Both seem to work as expected. Perhaps we have something different with our environments, either mouse drivers or one of the helper libraries. Do you have all updates applied to 12.04 (using apt-get update and the Update Manager)?

You sent in a specific user's bcsupport package. In their environment, when they run BC4, the issue reproduces, correct?