In OS X El Capitan, Apple introduced a feature called "Split View" that allows you to put two fullscreen apps side by side. Normally when you click and drag the middle dividing line one of the two apps will blur. If you hold Option while moving the divider neither window will blur.
How can I make this the default behavior, so I don't have to hold Option when resizing?
Resizing windows in split view no longer blurs one window as of macOS Catalina.