Currently 'Select All' selects all unlocked Layers. The only way to copy the content of the active layer is to lock all others first. This most basic operation is very time consuming and error prone.
It has been mentioned here before, but no bug report seems to have been made:
Needed for combining, making shapes and fills.
It would best be implemented with a new function, to avoid confusion.
Alternativly it could also be implemented by using Select All together with shift.
Valid enhancement request. Status set to NEW. Platform to All/All. Severity to enhancement.
*** Bug 97891 has been marked as a duplicate of this bug. ***
On Hackfest Hamburg, April 2018, we have decided to start a project for improving handling of layers and internal structure. In case you start to work on this issue get in contact with Regina (firstname.lastname@example.org) or Armin (Armin.Le.Grand@me.com).