+++ This bug was initially created as a clone of Bug #1497599 +++ We are currently doing layouts during the browser.xhtml document parse, which causes strange interactions with XBL construction where we have bindings that get created in the middle of parse. Those bindings are generally expecting the DOM to be mostly-ready when constructed (i.e. after MozBeforeInitialXULLayout). Based on discussion starting around https://bugzilla.mozilla.org/show_bug.cgi?id=1497599#c15, I think we can more closely match XUL document behavior if we hide the documentElement during parse (which should prevent us from doing layouts and constructing XBL). This needs a bit more testing, but initial try pushes indicate this may also fix some tests (since instead of waiting for DOMContentLoaded to run gBrowserInit.onBeforeInitialXULLayout we could do it immediately before unhiding the documentElement)

By hiding the documentElement with display:none and then waiting to show it until readystatechange, we can avoid inadvertently constructing XBL bindings as a result scripts running during parse like connectedCallback. This more closely matches XUL document behavior, where the initial layout doesn't happen until everything is parsed and loaded. It also lets us call gBrowserInit.onBeforeInitialXULLayout actually before layout happens.

Pushed by bgrinstead@mozilla.com: https://hg.mozilla.org/integration/autoland/rev/bb7c6b3e49c7 Prevent layouts during parse of browser.xhtml;r=Gijs,emilio