An option to display windows from the current screen only would be a nice addition.
Duplicate of bug 1053202 (which is already fixed and released).
I don't think that's right. By screen, I mean monitors. Setting the current workspace setting does not hide windows on other monitors for me.
lxqt-panel has this feature and it works perfectly outside of a desktop environment. Plank can probably implement it the same way. This goes hand in hand with #1259816
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Duplicate of bug 1053202 (which is already fixed and released).