Nightly workbench Fit gui on Ubuntu

Just tried out the nightly build on Ubuntu.
In the workbench when I open the Fit gui I cannot expand the window.
The slide in the gui does work.

I’m not sure I quite understand. Do you mean part of one of the interfaces? Such as the F(q) Fit tab of the Indirect Data Analysis interface?

I plot a spectrum direct from the workspace and click on the Fit button.
On Ubuntu I can’t expand the window.
On Windows I can’t move the divider - it springs back to default position.

I’ve installed the Nightly 27/03/2020 on Ubuntu and clicked fit on a plotted spectrum.
I can drag out the overall plot window, and I can move the divider between the Fit Property Browser and the plot.

But, I cannot maximise (or minimise) this window on Ubuntu! This was due to a change for version 5.0 which keeps plots on top of the main Mantid Workbench window. The divider springing back to its default on Windows is odd!