There are two possible reasons I can see for this happening.
First, Eclipse running in Ubuntu by default uses the GTK3 graphics API. This does not work properly. You have to disable it until the suite of issues around this is resolved.
The following has some information about this: How to work around Eclipse Luna/GTK bug 339174 (change to not use oxygen) .
Second, you didn't mention this, but I see this particular symptom in my Ubuntu VM running in VirtualBox in full-screen mode. You can tell if this is happening if you hover your cursor at the bottom right corner of a resizable window, and you see that the two-way drag icon doesn't appear until you're quite a few pixels down and to the right of the actual corner. Until this is fixed, you can mitigate this by going out of full-screen mode (right-ctrl-f, most likely) and then back in.