Issue 4101 - Prevent runaway MutationObservers

include.preload.js

Index: include.preload.js
diff --git a/include.preload.js b/include.preload.js
index 949d039663f9130843b4d8d0a3ddfcb434522066..c88f73f796c9ee8b27eaa30d54046471a134e0b6 100644
--- a/include.preload.js
+++ b/include.preload.js
@@ -150,10 +150,17 @@ function checkCollapse(element)
     {
       function collapseElement()
       {
+        var propertyName = "display";
+        var propertyValue = "none";
         if (element.localName == "frame")
-          element.style.setProperty("visibility", "hidden", "important");
-        else
-          element.style.setProperty("display", "none", "important");
+        {
+          propertyName = "visibility";
+          propertyValue = "hidden";
+        }
+
+        if (element.style.getPropertyValue(propertyName) !== propertyValue ||

[Sebastian Noack, 2016/08/10 12:55:57]

Nit: AS per Mozilla's coding practices we perfer != over !==.

[kzar, 2016/08/11 13:53:30]

Done.

+            element.style.getPropertyPriority(propertyName) !== "important")
+          element.style.setProperty(propertyName, propertyValue, "important");
       }
 
       if (collapse && !element._collapsed)

[Sebastian Noack, 2016/08/10 12:55:57]

With your changes to collapseElement(), the _collapsed guard logic became
redundant. For reference, it protects against a similar recursion, where <input
type="image"> elements load thei image again when the styles got changed,
causing it to be blocked again, triggering element collapsing again, and so on.

[kzar, 2016/08/11 13:53:30]

Done.