So I just tried Eclipse Mars.2 in Ubuntu MATE (in a VM).... It is quite broken - but not if launched with GTK2, with GTK2 it works ok.
With GTK3, it is buggy to the point of being unusable - closing views leaves a ghost view pane in the workbench page. Certain dialogs don't work because tree/list contents don't show up, so you can't select items, etc.
Is this normal? When you mentioned those problems with GTK3 on Ubuntu I thought it was "just" theming and visual artifact problems, but that otherwise one could still use the IDE normally.