I've noticed that dedent seems broken for mult-line selections. I'm attaching a python file with som indentation structure. All tabs.
You can see the indentation is wrong from 3 to 21 inclusive. As if to fix, select from the beginning of line 3 to the end of line 21. Use the Edit->Indentation->Dedent. Notice that dedenting stops after line 13. Lines 14-21 remain at their previous indentation.