Zen Mode is a Hammerspoon script, not an app: one hotkey (Option + Cmd + Z) glides the focused window to center, grows it to full height, and fades the Menu Bar, Dock, and everything else to black. Because it operates at the window-manager level instead of inside a single app, the same keystroke works identically in Terminal, a browser, or a PDF reader — unlike the zen modes built into individual editors, which only ever cover that one app. Press Esc or the hotkey again to restore the window to its exact prior position and size.