Use cross-browser document.activeElement instead of (non-IE) event.target.activeElement

Conflicts:
	CONTRIBUTORS.rst

CHANGELOG.txt

@@ -55,6 +55,7 @@ Changelog
 ~~~~~~~~~~~~~~~~~~
 
  * Fix: Paste / drag operations done entirely with the mouse are now correctly picked up as edits within the rich text editor (Matt Fozard)
+ * Fix: Logic for cancelling the "unsaved changes" check on form submission has been fixed to work cross-browser (Stephen Rice)
 
 
 1.4.4 (10.05.2016)

CONTRIBUTORS.rst

@@ -137,6 +137,7 @@ Contributors
 * Juha Yrjölä
 * Bojan Mihelac
 * Robert Moggach
+* Stephen Rice
 
 Translators
 ===========

docs/releases/1.4.5.rst

@@ -14,3 +14,4 @@ Bug fixes
 ~~~~~~~~~
 
  * Paste / drag operations done entirely with the mouse are now correctly picked up as edits within the rich text editor (Matt Fozard)
+ * Logic for cancelling the "unsaved changes" check on form submission has been fixed to work cross-browser (Stephen Rice)

wagtail/wagtailadmin/static_src/wagtailadmin/js/core.js

@@ -67,7 +67,7 @@ function enableDirtyFormCheck(formSelector, options) {
     window.addEventListener('beforeunload', function(event) {
         // Ignore if the user clicked on an ignored element
         var triggeredByIgnoredButton = false;
-        var $trigger = $(event.target.activeElement);
+        var $trigger = $(event.explicitOriginalTarget || document.activeElement);
 
         $ignoredButtons.each(function() {
             if ($(this).is($trigger)) {