diff --git a/webroot/rsrc/js/core/behavior-device.js b/webroot/rsrc/js/core/behavior-device.js
--- a/webroot/rsrc/js/core/behavior-device.js
+++ b/webroot/rsrc/js/core/behavior-device.js
@@ -26,6 +26,19 @@
       var v = JX.Vector.getViewport();
       var self = JX.Device;
 
+      // Even when we emit a '<meta name="viewport" ... />' tag which tells
+      // devices to fit the conent to the screen width, we'll sometimes measure
+      // a viewport dimension which is larger than the available screen width,
+      // particularly if we check too early.
+
+      // If the device provides a screen width and the screen width is smaller
+      // than the viewport width, use the screen width.
+
+      var screen_width = (window.screen && window.screen.availWidth);
+      if (screen_width) {
+        v.x = Math.min(v.x, screen_width);
+      }
+
       var device = 'desktop';
       if (v.x <= self._tabletBreakpoint) {
         device = 'tablet';