Not sure if this is correct spot, but I am having issues when connecting to my RB750G running 5.8RC. Here is a screen shot showing what it looks like.
ROS Ver: 5 RC8
WinBox Version: 2.2.18

It seems to only happen when connecting to ROS 5 boxes.

I have tried clearing the cache via winbox and manually.
Also tried w/ w/o secure and "load last session"

I get the same problem. Right click the grey space between the windows task bar the and menu on the left and add a gauge (any gauge), and it expands to use the full window size.

Hey fewi: What exactly do you mean by "add a gauge"

Right click the empty gray space between the Windows title bar ("admin@66.78.100.108") and the menu on the left (the "Interfaces" button). You get a menu that says "Add CPU", "Add Memory", and "Add uptime". Those are little gauges that show resource information in that space. Adding a gauge or removing a gauge resizes the window correctly.

If that's still unclear I can post a screenshot.

I got it now. Thanks again fewi.

Whats this? Maybe this is your problem, some plugin?

Normis: That is from a screen sharing / remote support program I use called TeamViewer. It can allow you so share an app remotely (among other things).

I have tried with and without, but so far today it is working both ways.

Whats this? Maybe this is your problem, some plugin?

Yes!

It IS TeamViewer causing the issue. TeamViewer > Options > Advanced > QuickConnect > Configure.. > Add winbox.exe as an application not to run QuickConnect for.

Solves the problem immediately.

Excellent find guys.

EDIT : Submitted to TeamViewer as bug.