The EWMH specification is very bogus about when a window should be decorated. This means that there is no existing policy when a window will be decorated by the window manager and when not. This document fixes this lack for the KDE Plasma Desktop workspace by clearly defining when a window is decorated. This policy applies since version 4.8.
In general all windows managed (windows without override-redirect) by the Window Manager (KWin) are re-parented into a Server-Side Decoration unless:
The KDE Plasma Workspaces guarantee a consistent look and feel for all windows. We want to provide the best possible user experience for each of our workspaces. We have different policies regarding when the server side decoration is shown for the different form factors. We don't want that the application developers have to adjust their application to work correctly on form factor desktop, netbook and tablet. Only by enforcing Server-Side decorations the workspaces can ensure that the correct user experience is implemented on each form factor.
Even with Server-Side decoration it is possible to influence the look and feel of the decoration. Please get in touch with either the KWin or Oxygen developers for help.
Imagine each application would have an awesome idea how to handle window management. This would result in a setup where each window is different and broken. Innovation is important and we don't want to stop innovation. But innovation has to be done correctly. This means the workspace has to innovate how windows behave. Please get in touch with the KWin, Plasma and Oxygen developers and explain what you want to achieve. We can help you.