VirtualBox 5.2.12-122591
Host: Ubuntu 18.04 running kernel 4.15.0-23-generic 64-bit
Host: Slackware 14.2+ (aka Current) running kernel 4.17.1-burdi64 64-bit
Guests: A Slackware-based live ISO running kernel 4.17.1-burdi64 64-bit - userland 32-bit c.q. 64-bit - no guest additions, no compiler etc.
The decorations of a just-opened window (e.g. of an xterm or a file manager) are not responsive: it is not possible to e.g. close the window via its URH "X". The panel with its launchers most of the time stays responsive, but I have seen it freeze at times as well.
[Auto]logging in on an alternative console (e.g. Win+F2 > Enter > Win+F7) unfreezes things: the window can be closed via its URH "X" again.
I have seen these symptoms since kernel 4.17. The same live ISO with e.g. kernel 4.16.13 is OK.
Testing with the 5.2.13-122890 and 5.2.97-122902 test builds gave the same symptoms.
Edit 1 - Jun 14, 2018: Testing the same ISO on "real" hardware does not show the symptoms.