Users of VirtualBox know that one of its nicest features is automatic guest screen resizing. Basically, every time you perform an action that causes the VM window to change its size (i.e. switch it between fullscreen and windowed mode, or change its size by dragging a border) the guest will receive an event that will cause it to change the screen resolution to match the effective window size. This means that, when fullscreen mode is entered or exited, you don’t have to open the guest’s screen settings and change the resolution manually. Proper useful!

Guest additions and their problems

This feature works by means of the guest additions, extra software that needs to be installed inside the guest and that provides tighter integration with the hypervisor.

Since VirtualBox 6.0, it is possible to choose the type of emulated graphics adapter:

  • VBoxVGA is a legacy adapter that is recommended for older OSes;
  • VBoxSVGA is recommended for modern Windows systems;
  • VMSVGA is recommended for modern Linux systems.

Now, this is the theory. It looks like that any recent Linux system should go with VMSVGA. However, I have encountered a number of issues with that adapter, including:

  • automatic screen resizing not working even when the guest additions are installed and exactly matching the version of the running hypervisor, if the additions come from prepackaged binaries of my distro. I needed to manually install them using the VirtualBox ISO to get them working;
  • even after fixing the point above, I got poor 2D performance in everyday desktop activity, such as very slow window dragging, portions of the screen showing as solid black and, more importantly, frequent VM crashes.

However, if I ignore the recommendation and keep using the legacy VBoxVGA adapter, things get better, much better. No crashes, black areas and windows can be dragged at a decent speed. But there is a price to pay: automatic screen resizing no longer works with Linux guests if they are not using the VMSVGA adapter. So you can basically choose between an unstable VM with working autoresize or a stable one without it.

Luckily, we are not forced to use VMSVGA to have working screen resizing.

Enter mplugd

Under a Linux VirtualBox machine with the guest additions installed, every time we resize the guest window, the system can immediately detect the new resolution. We can see that by calling xrandr after poking with the window size. The system alone, however, will not take any action when that happens. It is a VirtualBox additional component, VBoxClient, which listens for window size changes and adapts the screen to follow. As said above, this only works if using the VMSVGA adapter.

However, since the window size can be read by any system tool, couldn’t we use a different tool to listen for resolution changes in place of VBoxClient and then call xrandr to change the screen resolution on the guest?

The answer is yes, and such a tool already exists: mplugd. It a generic event listener based on plugins, which matches events against rules to execute when a certain thing happen. You can write rules such as “when event X happens, execute script Y”.

Since it already supports X events via a dedicated plugin, it can be put into immediate use by telling it to adjust the screen resolution when a screen change event happens.

Note: mplugd is written in Python 2, which has been retired at the beginning of 2020. Nevertheless, this utility is extremely useful and still working, so until it is ported to Python 3 or something better comes out, it is a good way to work around VirtualBox problems.

Let’s see how we can install and configure it. The following sections show how to do it on an Arch Linux system, but the general principles hold for any distribution.

Installation and configuration

Under Arch Linux, mplugd is available via the AUR, so you can use your favourite helper to install it, or do it manually. To handle X events, it needs python2-xlib, which must be installed separately. python2-setuptools are also needed:

pacman -Syu python2-setuptools
yay -Su mplugd-git python2-xlib

Once installed, rule definitions can be placed globally under /etc/mplugd/action.d or locally under $HOME/.mplugd/action.d. Since we want screen resizing to work for every user, we go for the first location.

# Edit /etc/mplugd/action.d/vboxresizing.rules
[rule vboxresizing]
on_type=OutputChangeNotify
true_exec=xrandr --output %event_name% --auto

We are asking mplugd to invoke xrandr every time a video output changes. The name of the output to be acted upon (i.e. VGA-1) is derived from the event and made available for substitution as %event_name%.

Last, we must ensure that mplugd is started every time a user logs in. Since the package doesn’t come with a systemd unit, we will put a global desktop entry for it under /etc/xdg/autostart/, so that every user gets it for free:

# Edit /etc/xdg/autostart/mplugd.desktop
[Desktop Entry]
Name=mplugd
NoDisplay=True
Exec=/usr/bin/mplugd

Starting from the next login, the guest screen should automatically resize, even when using the VBoxVGA adapter.