Whenever I double click on a MIDI clip in the clips pane, it automatically open that track in the Piano Roll.
You can open the Piano Roll view by any of these methods:
1. Select the track you want to edit and use the Views > Piano Roll View command or press ALT+3.
2. Double-click a MIDI clip in the Clips pane (you can change this option with the Track view Options > Click Behavior > Double-Click > MIDI Clips command).
If the Piano Roll opens up in the Multidock, and you wish to float and resize it, just right-click on the Piano Roll tab and select "undock".
And just clicking on the tracks number at the left side of the track strip "selects" that track, but does not change the Piano Roll tracks. It is now selected for echo, editing, processing, etc.
Also check out the "Tracks" button at the top edge of the Piano Roll window. This menu lets you choose the tracks viewed in Piano Roll. Just clicking on the tracks above in the track view does not change the track viewed in Piano Roll. That has a separate menu for track selection. Maybe that wasn't obvious, but it's the way it works.