Started with switching to 5.0.*, in effect with latest virtualbox-ose-5.1.6_1.
Auto resizing of guest display window (which is enabled by default) resizes it always to weird size like 1198x96 pixels and moves tightly to top edge, putting the window caption out of host screen. Manual resizing works no more than for 1 second, until automatic resizing triggers.
Disabling this auto resizing in window menu stops the effect but causes need to resize manually.
Selection of guest screen resolution or pressing Host+A causes one-shot resizing to the same invalid size.
FreeBSD 10.3-RELEASE on amd64
X Window: xorg-7.7; server package: xorg-server-1.14.7_5,1
WM: WindowMaker windowmaker-0.95.6_4
Qt packages all of 4.8.7 version with minor revisions
The effect seems not depending on guest system: the same with Ubuntu14.04, FreeBSD9, FreeBSD11.