>>> [!note] Migrated issue <!-- Drupal.org comment --> <!-- Migrated from issue #3594959. --> Reported by: [tim bozeman](https://www.drupal.org/user/2241356) >>> <h3 id="summary-problem-motivation">Problem/Motivation</h3> <p>Edit Mode sidebars were assembled imperatively in <code>Edit::buildSideBars()</code> and the client tracked a single active sidebar at a time. This had several limits:</p> <ul> <li>Only one panel could be open across the whole UI, so a left-rail tool palette and a right-rail panel could not be shown together.</li> <li>Panels were effectively tied to Edit Mode; an always-on panel (for example an AI chat available on every page, including anonymous and non-edit routes) had nowhere to live.</li> <li>Tools contributed panels through <code>buildLeftSideBar()</code> / <code>buildRightSideBar()</code>, which the mode hard-wrapped, so panels were not independently discoverable or reusable.</li> </ul> <h3 id="summary-proposed-resolution">Proposed resolution</h3> <p>Introduce a discoverable <code>#[Sidebar]</code> plugin type, move panel assembly to <code>NavigationPlusUi</code>, and make the client <code>SidebarManager</code> track one active panel <em>per side</em>.</p> <ul> <li>Add the <code>Sidebar</code> attribute, <code>SidebarInterface</code>, <code>SidebarPluginBase</code> and <code>SidebarPluginManager</code>. Two axes govern a panel: <code>applies()</code> decides DOM presence, while the <code>mode</code>/<code>tool</code> binding plus the <code>{id}_sidebar</code> open/close cookie decide visibility.</li> <li>Move sidebar building from <code>Edit::buildSideBars()</code> into <code>NavigationPlusUi::buildSidebars()</code>, <code>buildSidebarPanel()</code> and <code>isSidebarVisible()</code>. <code>buildSidebarPanel()</code> is public so dynamic, per-request panels (e.g. edit_plus per-entity Change forms) can be rendered for AJAX replacement into the stable <code>navigation-plus-{side}-sidebar</code> wrapper.</li> <li>Port the built-in Settings and Notifications panels to <code>#[Sidebar]</code> plugins. Their toggle buttons are pulled from the plugins into the mode top bar; an always-on (mode-independent) panel instead gets a fixed-position floating launcher.</li> <li>Extract <code>ModeState</code>, a lightweight request-derived mode lookup, so the cookie-auth-critical messenger decorator no longer depends on the now heavier <code>NavigationPlusUi</code> (which pulls in the plugin managers), avoiding a container circular reference.</li> <li>Dispatch <code>LoadEditablePageEvent</code> when the editable page AJAX response is built so modules can attach their own commands.</li> <li>Client: <code>SidebarManager</code> exposes <code>openSidebar(id)</code> / <code>closeSidebar(id)</code> / <code>closeSide()</code> / <code>closeAll()</code> / <code>closeEphemeral()</code> as the single open/close path; type and side are read from the panel element. A shared <code>CLOSE_BLOCKED</code> sentinel and a <code>[data-sidebar-close]</code> behavior let a panel own its close control. Persistent panels survive leaving Edit Mode.</li> </ul> <h3 id="summary-ui-changes">User interface changes</h3> <ul> <li>Left- and right-rail panels can now be open simultaneously (one active panel per side, mutually exclusive within a side).</li> <li>Always-on panels render a floating launcher button outside Edit Mode.</li> </ul> <h3 id="summary-api-changes">API changes</h3> <ul> <li><strong>BC break:</strong> <code>ToolInterface::buildLeftSideBar()</code> and <code>buildRightSideBar()</code> are removed. Tools that need a panel now ship a <code>#[Sidebar]</code> plugin.</li> <li>New plugin type <code>#[Sidebar]</code> (<code>Plugin/Sidebar/</code>), with manager service <code>plugin.manager.navigation_plus_sidebars</code> and <code>hook_navigation_plus_sidebar_info_alter()</code>.</li> <li>New <code>navigation_plus.mode_state</code> service (<code>ModeState</code>); <code>EditModeMessenger</code> now depends on it instead of <code>NavigationPlusUi</code>.</li> <li><code>NavigationPlusUi</code> gains public <code>buildSidebarPanel()</code>, <code>isSidebarVisible()</code>, <code>getActiveTool()</code> and <code>getToolPlugins()</code>.</li> <li>New <code>LoadEditablePageEvent</code>.</li> <li>Shared sidebar JS moved to the <code>navigation_plus/sidebar</code> library; the <code>SidebarManager</code> API changed from a single active sidebar to per-side tracking.</li> </ul>