Bug 1547351 - Set renderroot of extension popup on browser element r=mattwoodrow

I'm not sure when/if this stopped working, but it appears the -moz-stack
display of the stack element doesn't send us through the admittedly
brittle pathways that allow us to create nsDisplayRenderRoots. This
brittleness is a consequence of copying the use of the "layer"
attribute and is discussed in the core document splitting bug. It's
outside the scope of this bug to fix that.

Differential Revision: https://phabricator.services.mozilla.com/D31006

--HG--
extra : moz-landing-system : lando

browser/components/extensions/ExtensionPopups.jsm

@@ -239,7 +239,6 @@ class BasePopup {
 
     let stack = document.createXULElement("stack");
     stack.setAttribute("class", "webextension-popup-stack");
-    stack.setAttribute("renderroot", "content");
 
     let browser = document.createXULElement("browser");
     browser.setAttribute("type", "content");
@@ -257,6 +256,7 @@ class BasePopup {
     if (this.extension.remote) {
       browser.setAttribute("remote", "true");
       browser.setAttribute("remoteType", E10SUtils.EXTENSION_REMOTE_TYPE);
+      browser.setAttribute("renderroot", "content");
     }
 
     // We only need flex sizing for the sake of the slide-in sub-views of the