When you have part of the waveform selected and you set loop mode (forward/pingpong…), I assume that it sets the start and endpoint to selection.
I have the feeling that it used to be like this?
There used to be a couple of set loop buttons that did this. Was nice. And the fine loop editor. Wonder what’s up with that?
Oh, I forgot to say that the shortcut still works.
ctrl+shift+L 
Set loop to selection and rock and roll. Or find it in the right clicks.