I was trying to write some new unit tests for <settings-main> (specifically about "search within settings"). I am hitting a problem with the following method at [1]

/** @override */
attached: function() {
  this.scroller = this.domHost && this.domHost.parentNode.$.mainContainer;
},

MainPageBehavior currently assumes that elements implementing that behavior reside inside a paper-header-panel, see [2]. While in a unit test, this assumption is broken and a runtime error is thrown (see attachment). I also recall that when we transitioned from various paper-* elements to their equivalent app-* elements, I could not figure out how to remove the paper-header-panel parent, without the UI being very broken.

I am primarily looking for a way to make <settings-main> unit testable, and secondarily for a way to move away from paper-header-panel, since we already transitioned other parts of the app to the app-layout elements (for example app-drawer).

[1] https://cs.chromium.org/chromium/src/chrome/browser/resources/settings/settings_page/main_page_behavior.js?l=44
[2] https://cs.chromium.org/chromium/src/chrome/browser/resources/settings/settings_ui/settings_ui.html?l=131-135