Comment 10 for bug 1815693

I might have found the same or a similar problem and a simple way to reproduce this issue using glxgears. This works even on some real hardware systems (depending on the graphics driver which is used).

export GALLIUM_DRIVER="llvmpipe"
export LIBGL_ALWAYS_SOFTWARE="true"
glxgears -info

When glxgears is using llvmpipe or softpipe as renderer, it should be reproducible. Just grab a corner of the glxgears window and quickly move it around while watching Xorg memory (e.g. with "top -p $(pgrep Xorg)") which should quickly start to grow. In my VirtualBox and vSphere systems, llvmpipe was used by default, so the exports are not necessary and the problem always occurs.

As a workaround for VirtualBox, you can try to enable 3D-acceleration (VM settings -> Display -> Screen). For me, the Xorg memory stayed constant when 3D acceleration was activated (unless I forced the software rendering for glxgears).

To me this looks like a problem in Mesa as I have seen the same behavior on Ubuntu 18.04 (with stock Mesa 18.2.2 and also using the Ubuntu-X team PPA with Mesa 18.3.3), Fedora 29 and Debian 9.