I usually have my Emacs frame split into windows. I'll select something in one window and then switch to another buffer. When I do this Emacs removes the active-region highlghting in the original window. I'd like to keep it highlighted so I know what I was focused on while working in the other window.
highlight-nonselected-windows is a variable defined in xdisp.c. Its default value is nil. A non-nil value means highlight region even in nonselected windows. You can customize this variable, e.g.:
(setq highlight-nonselected-windows t)
