1. Open Unity
2. Make sure Unity is not fully maximized
3. Undock any panel, drag it around in various combinations with releasing, clicking on other windows and dragging then, etc

Reproduced with: 5.6.3p1, 2017.2.0f3, 2018.1.0a3

Expected result: no panel ever gets hidden under the Taskbar
Actual result: undocked panel sometimes gets obstructed by the Taskbar

Note: As seen in the video, almost a 100% repro happens when the panel is picked up immediately after leaving Window preview on the Taskbar without clicking elsewhere.
Note 2: This is likely to be a not a proper bug. Filing it out in case any related problems ever pop up.