i've two screens, the right one has an orizontal panel; while the left one has a vertical panel.
It seems that the left vertical panel is surrounded by a margin that makes impossivle to place widgets near the panel.
The "bug" is that that margin is too wide; more precisely it is wide exactly as the panel; this means that the bigger the panel; the bigger will be the margin.
See screenshot attached for details; you can see i'm trying to move the folderview widgets but plasmashell highlights what will be the folderview position at the end of the drag operation; and clearly it wants to place it very far from the vertical panel.

Reproducible: Always

Steps to Reproduce:
1.have a multihead configuration with two screens and an entire desktop spawned on horizontal.
2. make the right screen the primary one
3. place panels on the screen as seen in the attached screenshot.
4. try to place a widget near the rightmost, vertical, panel


Actual Results:  
see that there is a huge margin around the rightmost, vertical panel.
Notice also that if you enlarge the vertical panel, the margin will be even bigger.

Expected Results:  
The margins have to be smaller, and should not be big as the panel width.