--- a/browser/base/content/tabview/drag.js
+++ b/browser/base/content/tabview/drag.js
@@ -200,17 +200,17 @@ Drag.prototype = {
     var swb = this.safeWindowBounds;
     var update = false;
     var updateX = false;
     var updateY = false;
     var snappedTrenches = {};
 
     var snapRadius = (Keys.meta ? 0 : Trenches.defaultRadius);
     if (rect.left < swb.left + snapRadius ) {
-      if (stationaryCorner.indexOf('right') > -1)
+      if (stationaryCorner.indexOf('right') > -1 && !assumeConstantSize)
         rect.width = rect.right - swb.left;
       rect.left = swb.left;
       update = true;
       updateX = true;
       snappedTrenches.left = 'edge';
     }
 
     if (rect.right > swb.right - snapRadius) {
@@ -223,17 +223,17 @@ Drag.prototype = {
       } else if (!updateX || !Trenches.preferLeft) {
         rect.left = swb.right - rect.width;
         update = true;
       }
       snappedTrenches.right = 'edge';
       delete snappedTrenches.left;
     }
     if (rect.top < swb.top + snapRadius) {
-      if (stationaryCorner.indexOf('bottom') > -1)
+      if (stationaryCorner.indexOf('bottom') > -1 && !assumeConstantSize)
         rect.height = rect.bottom - swb.top;
       rect.top = swb.top;
       update = true;
       updateY = true;
       snappedTrenches.top = 'edge';
     }
     if (rect.bottom > swb.bottom - snapRadius) {
       if (updateY || !assumeConstantSize) {