Microsoft Windows enables you to activate a window's system menu using the Alt+Spacebar key combination and do so for an internal window ie (MDI) via Alt+- combinations. It is equivalent to clicking the icon on the application's title bar to activate the menu.
When I press Alt+Spacebar in KDE, the KRunner dialogue (Alt+Spacebar) pops up.
Does KDE have an equivalent for the Alt+Spacebar and if not how can I change the mapping from KRunner?
Unlike many other desktop environments, KDE Plasma doesn't use Alt+Spacebar as the default keyboard combination to bring up the client window menu to which you refer. One has to use Alt+F3 or one can click on the
Menu
button (circled) usually at the top left of the window's titlebar:If you wish to change things, you can via
Custom Shortcuts
. I've have an answer here that describes changing shortcuts which you may find helpful.I'm not going into specifics because keyboard shortcuts are a matter of personal choice and they also depend on what other shortcuts you have set up or intend to set up in the future.