The system tray tooltips will slide their position and fade their icon, but the position is based on the bottom-left and the necessary resize is instant. This means that moving the mouse between icons once the tooltip is visible looks terrible (especially if the cursor passes a couple of other icons on the way) - the box flickers all over the place, and shows the wrong icon fading to the correct one. The animations should either be extended to include the tooltip size, or disabled entirely. Reproducible: Always Steps to Reproduce: 1. Ensure a few tray icons are visible 2. Hover over an icon until the tooltip appears 3. Move the mouse to another icon
Resizing windows isn't really something you would want to do on an X server, even less animated. This could probably be fixed without looking terribly ugly some time with Wayland.
Wayland looks interesting. I'd suggest in the meantime that the animations are disabled entirely though; they're not adding anything until all the properties can be animated.
I posted something similar in KDE forums a while ago https://forum.kde.org/viewtopic.php?f=289&t=128612
While the tooltip implementation itself hasn't changed, we now have a morphing tooltips effect in KWin which nicely transitions the popup size as wel.