TamperMonkey script to disable merge options on Github branches

disable-merge-options-github-branches.js

// ==UserScript==
// @name         Disable merge options on Github branches
// @namespace    http://tampermonkey.net/
// @version      0.1
// @description  Avoid accidentally using the wrong merge option for specific branches. THIS SCRIPT NEEDS TO BE TWEAKED FOR YOUR PERSONAL NEEDS(more precisely "@match" column and "disabledBranchesByMethod").
// @author       Ivan Degtiarenko
// @match        https://github.com/<YOUR_ORGANIZATION>/<YOUR_REPOSITORY>/pull/*
// @icon         data:image/gif;base64,R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw==
// @grant        none
// @run-at       document-start
// ==/UserScript==

const disabledBranchesByMethod = [
    // ADD HERE: Branches for which "Create a merge commit" should be disabled. Example: ['branch-name-1', 'branch-name-2']
    [],
    // ADD HERE: Branches for which "Squash and merge" should be disabled. Example: ['branch-name-1', 'branch-name-2']
    [],
    // ADD HERE: Branches for which "Rebase and merge" should be disabled. Example: ['branch-name-1', 'branch-name-2']
    []
];

(function() {
    'use strict';

    // Use mutation observer as sometimes merge options widget loads later than the page.
    var observer = new MutationObserver(function(mutations, obs) {
        // Before this element is loaded function would spam errors to console.
        if (!document.getElementsByClassName("merge-message")[0]) {
            return;
        }
        let branchName = document.getElementsByClassName("gh-header-meta")[0].children[1].children[0].querySelector('.commit-ref').textContent;
        let mergeMethodsEnabled = [true, true, true];
        for (let i =0; i < 3; i++) {
            mergeMethodsEnabled[i] = !(document.getElementsByClassName("merge-message")[0].querySelector('.select-menu-merge-method').children[0].children[i].disabled);
        }
        for (let i =0; i < 3; i++) {
            mergeMethodsEnabled[i] = mergeMethodsEnabled[i] && (disabledBranchesByMethod[i].indexOf(branchName) === -1);
        }
        if (mergeMethodsEnabled.every(v => v === false)) {
            // Restrictions are too strict, falling back to using repo restrictions only.
            obs.disconnect();
            return;
        }
        // Going in reverse order, so the last click would be the click on the top-most available option.
        for (let i = 2; i >= 0; i--) {
            if (mergeMethodsEnabled[i]) {
                document.getElementsByClassName("merge-message")[0].querySelector('.select-menu-merge-method').children[0].children[i].click();
            } else {
                document.getElementsByClassName("merge-message")[0].querySelector('.select-menu').children[0].children[i].disabled=true;
                document.getElementsByClassName("merge-message")[0].querySelector('.select-menu-merge-method').children[0].children[i].disabled=true;

                // If disabled by branch-rule, add explanation note.
                if (disabledBranchesByMethod[i].indexOf(branchName) !== -1) {
                    let child = document.createElement('span');
                    child.className='unavailable-merge-method';
                    child.textContent='Not enabled for this branch';
                    document.getElementsByClassName("merge-message")[0].querySelector('.select-menu-merge-method').children[0].children[i].children[1].appendChild(child);
                }
            }
        }
        // Once we successfully disabled merge options, observer can be removed to ease the load.
        obs.disconnect();
    });
    observer.observe(document, {attributes: false, childList: true, characterData: false, subtree: true});
})();

Simple, but helpful. Thanks!
Line 31 was broken for me. Changing it as follow worked for me.

let branchName = document.getElementsByClassName("gh-header-meta")[0].children[1].children[0].querySelector('.commit-ref').textContent;

to

let branchName = document.getElementsByClassName("gh-header-meta")[0].children[1].children[0].querySelector('.commit-ref').textContent;

Not sure if this is helpful or not, but thought I'd share.

If you want to run the code when clicking on tabs, you can add the following code:
...
// @grant window.onurlchange
...

    const main = () => {
        // Use mutation observer as sometimes merge options widget loads later than the page.
        var observer = new MutationObserver(function(mutations, obs) {
        
        ...
        
        // Once we successfully disabled merge options, observer can be removed to ease the load.
            obs.disconnect();
        });

        observer.observe(document, {attributes: false, childList: true, characterData: false, subtree: true});
    }

    window.addEventListener('urlchange', (info) => main());
})();

Also preventing duplicated 'Not enabled for this branch' messages:

                    // If disabled by branch-rule, add explanation note.
                    if (disabledBranchesByMethod[i].indexOf(branchName) !== -1) {
                        const mergeElement = document.getElementsByClassName("merge-message")[0].querySelector('.select-menu-merge-method').children[0].children[i].children[1];

                        if (!mergeElement.getElementsByClassName('unavailable-merge-method').length > 0) {
                            let child = document.createElement('span');
                            child.className='unavailable-merge-method';
                            child.textContent='Not enabled for this branch';
                            mergeElement.appendChild(child);
                        }
                    }

Fix for the error in line 31:

let branchNameElement = document.getElementsByClassName("gh-header-meta")[0].children[1].children[0].querySelector('.commit-ref');
            if (!branchNameElement) {
                // Refined GitHub disabled extension:
                branchNameElement = document.getElementsByClassName("gh-header-meta")[0].children[1].children[2]
            }