[CentOS] Strange problem with CentOS 7 and guest additions in VirtualBox

Hi,

I'm having a very weird problem with CentOS as a VirtualBox guest. Let
me explain it step by step.

tl;dr: I can install VirtualBox Guest Additions and configure a
fullscreen 1280x1024 graphical resolution on a full install, but not on
a reduced install with a lightweight window manager.

Here's what I did.

1. Install vanilla CentOS 7 with KDE.

2. Boot to reduced 1024x768 resolution.

3. Install gcc, make and kernel-devel.

4. Build VirtualBox Guest Additions.

5. Reboot to full 1280x1024 resolution. ==> perfect

On a vanilla desktop installation, this works perfectly. Now here's my
second attempt.

1. Install minimal CentOS 7.

2. Install X11: yum groupinstall "X Window System"

3. Install WindowMaker from Nux repo.

4. Launch WindowMaker (startx) to reduced 1024x768 resolution.

5. Install gcc, make and kernel-devel.

6. Build VirtualBox Guest Additions.

7. Reboot and still have a reduced 1024x768 resolution. ==> meh

I made another attempt with Xfce instead of WindowMaker (yum autoremove
WindowMaker && yum groupinstall Xfce), but with the same result.

Any idea what's going on here?

Cheers,

Niki
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