Internal z-order: element types below staff lines, changed since v2.1
S3 - Major
In any score, change the color of a barline to white. It should appear as a white line on top of the staff lines, but it does not because it is behind the staff lines in the z-order. This applies to tie segments too. To see it clearly, change the color of the tie segment in the example file to white.
This is not an issue in 2.1. I have not tested earlier versions of v3.
I do not know what other elements might be drawing under the staff lines when they should not be, but hopefully we can sort out the complete list of them via this issue.
Technically we could say there is a workaround when it comes to this, although that in itself is no reason not to better the default values: A user can easily perform the function [Select all Similar] for barlines, ties, or whatever desired, and then increase the z-value a few clicks until above the staff.
The default z-order isn't necessarily intended to do good things if you start mucking with color etc, it's intended to produce the correct results in term of layout, which element gets selected when clicking, drag & drop behavior, etc. So to some extent it's expected you might need to alter default z-order here and there if you want to change color etc.
That said, I have nothing against optimizing this order, but it is definitely necessary to fully understand the reasons for the existing order and to do extensive testing to be sure nothing breaks from any change.
Was that change to "no" accidental? :) I hope so.
@Marc Sabatella I agree that we should understand this before mucking around with it. Do you know where to start looking in the code?