How to make a non-active window also react to mouse clicks immediately, rather than having it become only active?
Solution 1:
What application specifically? I tested with both Safari and Finder and found that clicks in an inactive window will both focus the window as well as activate a click event.
Solution 2:
Not exactly what you ask, but you can try Zooom/2 — one of its features is brining windows to front on hover after customizable time and that is just one of many features.
Also not excactly what you are asking: holding ⌘ and clicking background windows will fire click event without activating window.