Automatically pause the video when the mouse leaves the browser window or switches tabs, and automatically play the video when the mouse reactivates the browser window or switches back to the tab. `Ctrl+Alt+;` - AutoVideoPause Enable/Disable.

autopause.user.js

// ==UserScript==
// @name Automatically pause/resume video playback
// @namespace nemokaul
// @version 1.3
// @description Automatically pause the video when the mouse leaves the browser window or switches tabs, and automatically play the video when the mouse reactivates the browser window or switches back to the tab
// @author Nemo Kaul
// @match *://*/*
// @grant none
// @downloadURL https://gist.github.com/nemokaul/3c02aa3610c579a747bc36d125a14c00/raw/70ff8a5f7c710c2195027e34c24106d99c48a904/autopause.user.js
// @updateURL https://gist.github.com/nemokaul/3c02aa3610c579a747bc36d125a14c00/raw/70ff8a5f7c710c2195027e34c24106d99c48a904/autopause.user.js
// ==/UserScript==

(function() {
    var videoElement = null;
    var isPaused = false;
    var isScriptActive = false;

    // Get all video elements
    function getVideoElements() {
        return document.querySelectorAll('video');
    }

    // Pause video playback
    function pauseVideo() {
        if (!videoElement.paused) {
            videoElement.pause();
            isPaused = true;
        }
    }

    // Resume video playback
    function playVideo() {
        if (isPaused) {
            videoElement.play();
            isPaused = false;
        }
    }

    // Listen for mouse out of window events
    function handleMouseOut(event) {
        if (!isScriptActive) return;
        // Determine whether the mouse has left the entire browser window
        if (event.toElement === null && event.relatedTarget === null) {
            var videos = getVideoElements();
            if (videos.length > 0) {
                videoElement = videos[0];
                pauseVideo();
            }
        }
    }

    // Listen for mouse entering window events
    function handleMouseOver(event) {
        if (!isScriptActive) return;
        // Determine whether the mouse enters the entire browser window
        if (event.fromElement === null && event.relatedTarget === null) {
            playVideo();
        }
    }

    // Toggle script activation
    function toggleScript() {
        isScriptActive = !isScriptActive;
        if (isScriptActive) {
            showMessage("Video AutoPause is ON", "blue");
            window.addEventListener('mouseout', handleMouseOut);
            window.addEventListener('mouseover', handleMouseOver);
        } else {
            showMessage("Video AutoPause is OFF", "blue");
            window.removeEventListener('mouseout', handleMouseOut);
            window.removeEventListener('mouseover', handleMouseOver);
        }
    }

    // Show a message in the top right corner of the page
    function showMessage(message, color) {
        var messageElement = document.createElement('div');
        messageElement.style.position = 'fixed';
        messageElement.style.bottom = '10px';
        messageElement.style.left = '20px';
        messageElement.style.color = color;
        messageElement.style.fontSize = '15px';
        messageElement.style.fontWeight = 'bold';
        messageElement.style.backgroundColor = 'rgba(255, 255, 255, 0.7)';
        messageElement.style.padding = '20px';
        messageElement.style.border = '1px solid ' + color;
        messageElement.style.zIndex = '10000';
        messageElement.textContent = message;
        document.body.appendChild(messageElement);
        setTimeout(() => {
            document.body.removeChild(messageElement);
        }, 2000);
    }

    // Listen for the specific key combination to toggle the script
    // Alt+; -- Toggle script on or off
    document.addEventListener('keydown', function(event) {
        if (event.altKey && event.key === ';') {
            toggleScript();
        }
    });
})();