A Tampermonkey script to change the behavior of the copy branch button on GitHub PRs to copy some markdown instead

tampermonkey-gh-copy-branch-hook.js

(function() {
    'use strict';
    
    function copyMarkdown() {
        ...
        navigator.clipboard.writeText(markdown);
    }
    
    function addEventsToCopyElements() {
        // Get all elements with tag name clipboard-copy and class js-copy-branch
        const copyElements = document.querySelectorAll("clipboard-copy.js-copy-branch");

        // Add a replacement click event listener to each clipboard-copy element
        copyElements.forEach(element => {
            element.addEventListener("click", event => {
                event.preventDefault();
                // If the shift key is pressed, we want to allow the default behavior
                // and run the original click events
                if (event.shiftKey) {
                    // Run the original click event
                    oldClickEvents[element](event);
                }
                else {
                    // Otherwise, we want to copy the markdown to the clipboard
                    copyMarkdown();
                }
            })
        });
    }
    
    // A map of clipboard-copy elements to their click event listeners
    const oldClickEvents = new Map();
    
    // Hook addEventListener to intercept click events on clipboard-copy elements
    const originalAddEventListener = EventTarget.prototype.addEventListener;
    EventTarget.prototype.addEventListener = function(type, listener, options) {
        if (type === "click" && this.tagName === "CLIPBOARD-COPY") {
            // If the element is a clipboard-copy element, we want to store the click event listener
            oldClickEvents.set(this, listener);
        }
        else {
            // Add the event listener as normal
            return originalAddEventListener.apply(this, arguments);
        }
    };
    
    // Wait for the DOM to be loaded before adding events
    const observer = new MutationObserver((mutationList, observer) => {
        for (const mutation of mutationList) {
            if (mutation.type === 'childList') {
                addEventsToCopyElements();
                return;
            }
        }
    });
    observer.observe(document.body, { childList: true, subtree: true });
})();