Current behavior: sidebar overlays the note list
Proposed behavior: sidebar pushes the note list to the right
As far as I can tell from your description, the sidebar already does push the note list to the right in the Joplin mobile app. What is does not do is drag the notebook list as it is moved, but it reveals/uncovers it instead. So it doesn’t quite give the effect of moving as one unit.
Small but annoying: opening a note in view mode currently triggers keyboard popup on some devices
That’s a regression which has been fixed but is not released yet