When editing a panel and moving mouse around it - tooltips with widget names show. Tooltips closer to the edge make active window to shrink, leaving exact amount of pixels that the tooltips has in width. On mouse out, the window goes back to it's original form. Demo: https://drive.google.com/drive/folders/0B3ahLC5Mtqw3akRCZFg5cXZ2SHM?usp=sharing
Can reproduce.
Seems to be resolved for me with Plasma/KWin 5.10.
Yep, seems to be fixed. I'm also no longer able to reproduce.