I have two monitors:

Id:  67
Name:  "LVDS1"
Type:  "Panel (Laptop)"
Connected:  true
Enabled:  true
Primary:  true
Rotation:  1
Pos:  QPoint(1920,312)
MMSize:  QSize(344, 194)
Size:  QSize(1366, 768)

Id:  69
Name:  "HDMI1"
Type:  "Unknown"
Connected:  true
Enabled:  true
Primary:  false
Rotation:  1
Pos:  QPoint(0,0)
MMSize:  QSize(509, 286)
Size:  QSize(1920, 1080)

My larger HDMI monitor is first from left to right so plasma moves my panel to that monitor (whether that's a bug in itself is another issue, I suppose) but after that, pop-ups like the application launcher, hovering over applets, clipboard contents and the history of notifications pop using the old geometry. The attached screenshots show the problem.

Reproducible: Always

Steps to Reproduce:
1. Have plasma running on your main screen, with a appliation launcher applet.
2. Connect a new, larger screen to the left of your screen — plasma panels should relocate to the new screen.
3. Click on the application launcher applet.

Actual Results:  
Application pop-up appears somewhere in the middle of the screen.

Expected Results:  
Applicationi pop-up should appear right on top of the panel.

The third screenshot shows an overlay of the small scree over the large screen, showing that it seems to be that the pop up is using the old geometry, maybe a kwin bug?