--- a/browser/base/content/tabbrowser.xml
+++ b/browser/base/content/tabbrowser.xml
@@ -1812,21 +1812,21 @@
         }
 
         // Disable detach within the browser toolbox
         var eX = event.screenX;
         var eY = event.screenY;
         var wX = window.screenX;
         // check if the drop point is horizontally within the window
         if (eX > wX && eX < (wX + window.outerWidth)) {
-          let bo = this.arrowScrollbox.boxObject;
           // also avoid detaching if the the tab was dropped too close to
           // the tabbar (half a tab)
-          let endScreenY = bo.screenY + 1.5 * bo.height;
-          if (eY < endScreenY && eY > window.screenY)
+          let rect = window.windowUtils.getBoundsWithoutFlushing(this.arrowScrollbox);
+          let detachTabThresholdY = window.screenY + rect.top + 1.5 * rect.height;
+          if (eY < detachTabThresholdY && eY > window.screenY)
             return;
         }
 
         // screen.availLeft et. al. only check the screen that this window is on,
         // but we want to look at the screen the tab is being dropped onto.
         var screen = Cc["@mozilla.org/gfx/screenmanager;1"]
                        .getService(Ci.nsIScreenManager)
                        .screenForRect(eX, eY, 1, 1);

--- a/layout/reftests/w3c-css/received/reftest.list
+++ b/layout/reftests/w3c-css/received/reftest.list
@@ -96,17 +96,17 @@ fuzzy(0-107,0-1823) fails-if(webrender&&
 skip == css-multicol/multicol-gap-fraction-002.html reference/nothing.html
 fuzzy-if(winWidget||OSX||gtkWidget,0-204,0-1048) fuzzy-if(skiaContent,0-208,0-1048) == css-multicol/multicol-gap-large-001.xht css-multicol/multicol-gap-large-001-ref.xht
 fuzzy(0-225,0-920) fails-if(webrender&&cocoaWidget) == css-multicol/multicol-gap-large-002.xht css-multicol/multicol-gap-large-002-ref.xht
 fuzzy(0-204,0-930) fuzzy-if(skiaContent,0-208,0-930) fails-if(webrender&&cocoaWidget) == css-multicol/multicol-gap-negative-001.xht css-multicol/multicol-gap-002-ref.xht
 == css-multicol/multicol-height-001.xht css-multicol/multicol-height-001-ref.xht
 fails == css-multicol/multicol-height-block-child-001.xht css-multicol/multicol-height-block-child-001-ref.xht
 fuzzy(0-255,0-3762) == css-multicol/multicol-inherit-001.xht css-multicol/multicol-inherit-001-ref.xht
 fuzzy(0-135,0-1893) fails-if(webrender&&cocoaWidget) == css-multicol/multicol-inherit-002.xht css-multicol/multicol-inherit-002-ref.xht
-fails == css-multicol/multicol-inherit-003.xht css-multicol/multicol-inherit-3-ref.xht
+skip-if(cocoaWidget) fails-if(!cocoaWidget) == css-multicol/multicol-inherit-003.xht css-multicol/multicol-inherit-3-ref.xht # Bug 1432941
 fuzzy(0-97,0-264) == css-multicol/multicol-list-item-001.xht css-multicol/multicol-list-item-001-ref.xht
 fuzzy(0-73,0-1200) == css-multicol/multicol-margin-001.xht reference/ref-filled-green-100px-square.xht
 fuzzy(0-73,0-1200) == css-multicol/multicol-margin-002.xht reference/ref-filled-green-100px-square.xht
 fuzzy(0-243,0-3322) fuzzy-if(skiaContent,0-244,0-3322) == css-multicol/multicol-margin-child-001.xht css-multicol/multicol-margin-child-001-ref.xht
 fuzzy(0-255,0-4008) == css-multicol/multicol-nested-002.xht css-multicol/multicol-nested-002-ref.xht
 fuzzy(0-255,0-4109) == css-multicol/multicol-nested-005.xht css-multicol/multicol-nested-005-ref.xht
 fuzzy(0-225,0-13600) == css-multicol/multicol-nested-column-rule-001.xht css-multicol/multicol-nested-column-rule-001-ref.xht
 fuzzy(0-204,0-2463) fuzzy-if(skiaContent,0-208,0-2463) fails-if(webrender&&cocoaWidget) == css-multicol/multicol-nested-margin-001.xht css-multicol/multicol-nested-margin-001-ref.xht