Why can’t I seem to edit the video in and out points on my Mac with the waveform slider?

The original user interface design was created for iPad and touch input, and that translates well to a Mac touchpad, or a Magic Mouse with the gesture options that are available in System Settings > Mouse.

If you use a standard mouse and want to click-drag the in and out points you have to enable scroll bars. Go to System Settings > Appearance, and change Show scroll bars to Always. Small scroll bars will then appear in the waveform window so you can click-drag to adjust the in and out points.