Declarative

Buttons with paneltarget="id" open a <section panel> with that id. No JS required. The contents of the section become the panel body. When the panel closes, the contents are returned to the section.

<button paneltarget="inspector" paneltargetaction="toggle">Inspector</button>

<section id="inspector" panel
         panel-title="Inspector"
         panel-width="320" panel-height="280"
         panel-draggable panel-resizable panel-closable
         panel-focusable panel-stackable panel-titled panel-escapable>
  <p>Anything inside this section becomes the panel body.</p>
</section>