Apparently, the tests where we switch through the tabs of JMC, produces screenshots and compares them with a "golden standard", have changed recently. Instead of producing screenshot of a selected part of the screen, the whole desktop is included. This has the effect that the comparison fails.
We should fix so that only the correct part of the UI is included in the screen shot. It also has to be investigated why this change has happened.
(At first this seems to have affected only Linux, but in one case it has also affected Windows, so it seems like a more general problem.).