showSelectedSidebarInSwitcherPanel.uc.js

// ==UserScript==
// @name           Show Selected Sidebar in Switcher Panel
// @version        1.0
// @author         aminomancer
// @homepage       https://github.com/aminomancer/uc.css.js
// @description    For some reason, proton removes the checkmark shown on the selected sidebar in the sidebar switcher panel. (The one that pops up when you click the button at the top of the sidebar) This script simply restores the previous behavior of adding the [checked] attribute. On its own it won't do anything, since the CSS for adding checkmarks to the menu items has also been removed. Download uc-sidebar.css for the actual styling, or read it starting around line 120 if you want to make your own styles.
// ==/UserScript==

(function () {
    const builtInSidebars = ["Bookmarks", "History", "Tabs"];
    function init() {
        for (let type of builtInSidebars) {
            SidebarUI.sidebars.get(
                `view${type}Sidebar`
            ).buttonId = `sidebar-switcher-${type.toLowerCase()}`;
        }

        SidebarUI.selectMenuItem = function selectMenuItem(commandID) {
            for (let [id, { menuId, buttonId, triggerButtonId }] of this.sidebars) {
                let menu = document.getElementById(menuId);
                let button = document.getElementById(buttonId);
                let triggerbutton = triggerButtonId && document.getElementById(triggerButtonId);
                if (id == commandID) {
                    menu.setAttribute("checked", "true");
                    button.setAttribute("checked", "true");
                    if (triggerbutton) {
                        triggerbutton.setAttribute("checked", "true");
                        updateToggleControlLabel(triggerbutton);
                    }
                } else {
                    menu.removeAttribute("checked");
                    button.removeAttribute("checked");
                    if (triggerbutton) {
                        triggerbutton.removeAttribute("checked");
                        updateToggleControlLabel(triggerbutton);
                    }
                }
            }
        };
    }

    if (gBrowserInit.delayedStartupFinished) {
        init();
    } else {
        let delayedListener = (subject, topic) => {
            if (topic == "browser-delayed-startup-finished" && subject == window) {
                Services.obs.removeObserver(delayedListener, topic);
                init();
            }
        };
        Services.obs.addObserver(delayedListener, "browser-delayed-startup-finished");
    }
})();