Overview:
Dragging any window from the primary screen to the secondary screen through a vertical panel causes the window to vanish as it is moved. 

Steps to Reproduce:
1) Dual screens, primary screen on the right, secondary on the left. Both at 1080p resolution. 
2) A single panel, vertical, on the left edge of the primary screen (the shared edge between the two screens).
3) Drag a window from the primary (right) screen to the secondary (left) screen.


Actual Results:
The dragged window disappears when approximately 70% has moved into the secondary screen. The window in fact jumps off the lower edge of the visible screen area.

Expected Results:
The dragged window remains visible as it is moved into the second screen. 

Platform:
KDE Neon updated 21/5/2017. Plasma 5.9.5 Frameworks 5.34.0 Qt 5.7.1
Radeon HD 6850 on default drivers.