Try running it at a resolution the same as your desktop if none of these suggestions have worked.
To do that, find out your desktop resolution (1024x786 for example is HEIGHTxWIDTH) open console with the ~ key and type in these commands:
/set r_customheight 1024 (replace 1024 with your height)
/set r_customwidth 786 (replace 786 with your width)
/set r_mode -1
I run Ubuntu and freshly installed Trem less than a month ago and I had the same problem.