browser.devtools.panels.onHidden.addListener(listener)
browser.devtools.panels.onHidden.removeListener(listener)
browser.devtools.panels.onHidden.hasListener(listener)

Events have three functions:

addListener(callback)

Adds a listener to this event.

removeListener(listener)

Stops listening to this event. The listener argument is the listener to remove.

hasListener(listener)

Checks whether listener is registered for this event. Returns true if it is listening, and false otherwise.

callback

Function called when this event occurs. This function will be passed no arguments.

Create a sidebar pane, and log show and hide events.

function onCreated(sidebarPane) {

  sidebarPane.onShown.addListener(() => {
    console.log("Shown");
  });

  sidebarPane.onHidden.addListener(() => {
    console.log("Hidden");
  });

}

browser.devtools.panels.elements.createSidebarPane("My pane").then(onCreated);