To make a custom element fill the entire window, I am using @apply(--layout-fixed-left) and @apply(--layout-fixed-right). With this, all top/bottom/left/right are set correctly, but "position: fixed" is not applied.
Looking in iron-flex-layout.html, this is specified in:
--layout-fixed-bottom:,
--layout-fixed-left:,
--layout-fixed-right:,
--layout-fixed-top: {
position: fixed;
};
On a closer look, this is the single place where multiple mixins are specified in a single rule. So it might be an issue with how css rules are extracted from the mixins, when there's a group of them defined.
For now, the workaround is to have position: fixed next to the applied mixins.