I'm working on my first project in X3 (I use Sonar for 5+ years starting with 8.5). I may have changed some setting, and now:
a) No "Restore Strip Size" double arrow is visible on any track pane.
b) When I try re-sizing a track pane vertically by clicking on the horizontal line between this pane and another pane, and dragging, usually all track panes get re-sized proportionally; sometimes only the desired track pane gets re-sized, as I would expect.
If I create a new project and add a few tracks, all works as expected, i.e. not like in the aforementioned project.
So, is it a bug in Sonar or there is a a setting I have inadvertently changed?
 
Cheers

It's a feature called "Auto Track Zoom" introduced in X2. Hit Shift+Z to disable/re-enable it and see the difference.

Many thanks! You've saved me much confusion and time.