Virtualbox preview screen / screenshotpng stuck on black screen

[JEBjames]

Host: Windows 10 Pro 1709
VirtualBox: VirtualBox 5.2.12 /w extension pack + guest additions
Guest: Lubuntu 18.04 guest, 3D acceleration is off

Log file attached.

VirtualBox's mini "preview" of the guest screen contents is sometimes stuck all black. This same problem can be also be noticed when asking the vm for a screenshot:

"vboxmanage controlvm $vmName screenshotpng $imageFile"

Once stuck, both return an all-black image regardless of what's on the actual VM screen.

It's stuck returning an all-black image until I do either:
A)
- un-maximize the guest window (it becomes 1/4 screen with scroll bars. Note that it doesn't auto-resize the guest window no matter how long you wait.)
- I click and drag the virtualbox guest window border to resize it a bit bigger. At this point the guest window will resize itself. Usually this fixes it, sometimes I have to maximize the window again to make it come back.

B)
- In a console window in Lubuntu blank the screen with: "xset -display :0.0 dpms force off". Wait a few seconds. Press a key to unblank the screen. The preview window will work again.

C) the guest is rebooted

If the guest vm is still minimized with an lxterminal window up I can still use vboxmanage to feed keystrokes into the guest. So communication with vboxmanage isn't completely broken, just the sending of the virtual screen image.

Related note...while monkeying around with I think resizing/maximizing I was able to once get the actual guest screen (not the preview window) to be solid black. This only happened once. And doing A) fixed it.

I have run the guest for hours with a fullscreen app that blocks the screen saver and never changes video mode. This combination hasn't triggered the problem yet.

It seems to be some combination of changing the video mode, vm being idle, or screen (un)blanking that triggers it.

It happens semi-regularly with Lubuntu 18.04 if I leave the guest idle for a bit and do some work in another window and then come back.

If it helps, the Lubuntu 18.10 daily beta will 100% of the time have the preview stuck on a black screen at the login screen. Once you log in it usually fixes itself, or doing the screen resize shuffle fixes it.

With Lubuntu 18.10, when it's stuck on black in the preview at the login prompt switching to a text mode console (control-alt-f2) unfreezes the preview window. Switching back to the graphical console (control-alt-f1) goes back to a black preview screen. Logging in and opening a terminal (control-alt-t) running "xset -display :0.0 dpms force off" will blank the screen. Wait a few seconds, then pressing a key to unblank the screen. Now the preview image works again.

[socratis]

It seems to be some combination of changing the video mode, vm being idle, or screen (un)blanking that triggers it.

Disable any screen saver in the guest. Disable any power saving mode in the guest. See if that helps...