There is this strange (non-standard) UI behavior where if you single click anywhere on the window area that has no controls (title , area around search box, etc), the UI unmaximizes the window. Is this a known issue? 21.0.85 on Mint 17.3 w/KDE. The standard behavior would be to unmaximize only on double click of the title bar. Kind of a major annoyance since I always run maximized and I'm always unmaximizing accidentally just trying to switch focus to Media Center or by trying to seek (clicking above and below the seek bar will unmaximize the window).
The UI behavior of MC21, up to this point as far as I know, has not been designed to conform to any specific Linux desktop UI (i.e., Cinnamon, Unity, Gnome, KDE, etc). It's a work in progress. For example, MC21 has, as yet, no ability to provide desktop notifications for things like song changes; no integration in the desktop UI for transport controls to control "play, pause, stop, next, previous" from any desktop, and no MPRIS D-Bus support to name a few. These things will, hopefully, come in the near future.
I use Ubuntu/Unity and when I click once on a blank area of the MC21 title bar nothing happens - the MC21 window stays the same; click twice and it maximizes to full screen; click twice again and it reverts to the original size window. A middle click (as I have it set in Ubuntu/Unity) should "shade" or minimize MC21, but it doesn't (to be fair, it doesn't in any other app either, it seems Ubuntu changed the "shading" effect to an automatic "minimize" to the Launcher). Clicking on the MC21 Search window brings up a small window allowing me to enter a search term or select from a common list of searches. I'm not sure why you are having a problem with that.
So each desktop environment, as set up by the user, is going to have a different affect on MC21. Or, looking at it another way, MC21 is not going to behave like a KDE app. I wouldn't call it a bug, although I can understand your frustration in expecting MC21 to behave like "Amarok" or some other app packaged and designed to work well in Mint/KDE.