I am actually using a big screen, 27" with 2560x1440 resolution. With such screen largeness, you really feel like having two screens merged together, so for example I started not maximizing windows, because otherwise it would be uncomfortable to go so far with the mouse pointer to interact with stuff. While right clicking on Plama panel -> Configure task manager, I see that the new window is opening always on the far right side of the screen. So for example if I was using the mouse on the left side of the screen, I will have to move it a lot to go to the opposite side to interact with the just appeared window. It would be nice to let new windows appear in the same area of mouse interaction
KWin is not responsible for placing context menus.
(This isn't about the context menu, but about the configuration window that appears.) You can override placement of windows using window rules or the window placement options in systemsettings.
Indeed, you can control for yourself how KWin positions new windows. There is a logic to placing a window in a corner, far from where you were probably looking: the next window you open after that will be far away on the other size of the screen, minimizing the chance of them overlapping (hence, this default window placement mode is called "Minimize Overlapping"). If you don't find that this behavior suits your tastes with that high resolution screen, you might consider switching to the "Centered" or window placement mode or a different one.