In Fullscreen, disable auto-hiding Mac menu bar
So it seems someone found a working solution, at last.

First,


close your VMs
Quit VirtualBox


then as root (or sudo ...) 


copy the file somewhere as backup, then edit (eg vi) /Applications/VirtualBox.app/Contents/Resources/VirtualBoxVM.app/Contents/Info.plist
and add the line shown in the middle


the line &lt;key&gt;LSUIElement&lt;/key&gt; &lt;string&gt;1&lt;/string&gt; to be inserted between LSCanProvideIMVideoDataSource and NSHighResolutionCapable

&lt;key&gt;LSCanProvideIMVideoDataSource&lt;/key&gt;    &lt;true/&gt;
&lt;key&gt;LSUIElement&lt;/key&gt; &lt;string&gt;1&lt;/string&gt;
&lt;key&gt;NSHighResolutionCapable&lt;/key&gt;          &lt;true/&gt;


save, then start VirtualBox again (I didn't have to log out/reboot).

The VM will not show anymore among the opened apps icons, but you can double click the VM in the VirtualBox to switch to it.

The whole post by Maestro is here.