Accentuation problems in editor


When I type accentuated chars in the editor, I get the correct accentuation, but right before it I get a sample of the single accentuation in question as well.

If I try to type não (no) for instance, I get n~ão every time.

Take a look in the screenshot:


I have the same problem on MacBook Air with OS X Yosemite (10.10.5), but not on PC with Windows 7.