diff --git a/core/misc/dialog/dialog.position.js b/core/misc/dialog/dialog.position.js
index 7678646..bc949a1 100644
--- a/core/misc/dialog/dialog.position.js
+++ b/core/misc/dialog/dialog.position.js
@@ -45,12 +45,15 @@
         // jQuery UI does not support percentages on heights, convert to pixels.
         if (typeof optionValue === 'string' && /%$/.test(optionValue) && /height/i.test(option)) {
           // Take offsets in account.
-          windowHeight -= displace.offsets.top + displace.offsets.bottom;
+          if (!event.data.settings.modal) {
+            windowHeight -= displace.offsets.top + displace.offsets.bottom;
+          }
           adjustedValue = parseInt(0.01 * parseInt(optionValue, 10) * windowHeight, 10);
           // Don't force the dialog to be bigger vertically than needed.
           if (option === 'height' && event.data.$element.parent().outerHeight() < adjustedValue) {
             adjustedValue = 'auto';
           }
+          adjustedOptions.modal = event.data.settings.modal;
           adjustedOptions[option] = adjustedValue;
         }
       }
@@ -74,7 +77,7 @@
    *   Altered options object.
    */
   function resetPosition(options) {
-    var offsets = displace.offsets;
+    var offsets = options.modal ? { left: 0, right: 0, top: 0, bottom: 0 } : displace.offsets;
     var left = offsets.left - offsets.right;
     var top = offsets.top - offsets.bottom;