13.0-RELEASE, virtualbox-ose-additions installed and enabled, seamless mode is working (no need to press the Host key to switch), but the window will not resize. I also got this notification upon login: " VBoxClient: Failed to register resizing support, rc=VERR_RESOURCE_BUSY "
(In reply to Guangyuan Yang from comment #0) What version of Virtualbox are you using? Additions are the same? If using Virtualbox 6.1.x, are you using the VBoxSVGA driver? In my tests it's the only one resizing works with. I know that you get a warning in the UI telling you to use VMSVGA controller, but that's bogus, please ignore that.
(In reply to Guangyuan Yang from comment #0) OTOH, if you're already doing as I said I'm sorry but I have no clue what the issue could be, it should work.
(In reply to Guido Falsi from comment #1) That is actually exactly the issue, thanks! I remembered setting it to VBoxSVGA, but it may get changed after an update of Virtualbox, and things just stopped working.
(In reply to Guangyuan Yang from comment #3) I'd like to add that the VirtualBox UI sometimes does change some settings depending on other ones. The graphic driver can be reset to their "suggested" one (VMSVGA for FreeBSD, which I would not suggest) when toggling some other switches. So it's always best to check the setting is actually the intended one before saving settings.