When hovering over a regular Button in DockWindow(and DockWindow is detached like on pic), there is that highlightFrame bug,
but when that same Button is hovered in DockWindow(in a dockable state) it's drawn like expected.
Other controls seems to be drawn as expected when hovered - docked or not.
That Button hover highlightFrame bug is also the same, when mouse goes out of control - that frame bug is exactly like when 1stTime hoveredOn.

See attached image.