js: Don't trigger beforerender or rendered for discarded content

A listener for `rendered` should not need to check if the content
really changed. Though, if such a listener is not triggered anymore,
`beforerender` listeners also must not be triggered, as they might
assume that the content is really being updated and their accompanied
`rendered` listener is triggered. (e.g. input-enrichment.js)

This might be breaking change and any `Behavior.renderHook` implementation
needs to be checked against it. Potentially also in third party modules.
As if such an implementation updates the container on its own,
`beforerender` listeners only have access to the updated container after
this change, while they had access to the original beforehand.

`rendered` listeners should not be that much affected, as for them the
change results in the same behavior as if no update has ever been
scheduled for the container.

fixes #5056

public/js/icinga/loader.js

@@ -873,7 +873,7 @@
                     }
                 })
             } else {
-                this.renderContentToContainer(
+                let rendered = this.renderContentToContainer(
                     req.responseText,
                     req.$target,
                     req.action,
@@ -882,6 +882,10 @@
                     req.autosubmit || autoSubmit,
                     req.scripted
                 );
+
+                if (! rendered) {
+                    req.discarded = true;
+                }
             }
 
             if (oldNotifications) {
@@ -905,6 +909,14 @@
                 req = reqOrError;
             }
 
+            req.$target.data('lastUpdate', (new Date()).getTime());
+            delete this.requests[req.$target.attr('id')];
+            this.icinga.ui.fadeNotificationsAway();
+
+            if (req.discarded) {
+                return;
+            }
+
             // Remove 'impact' class if there was such
             if (req.$target.hasClass('impact')) {
                 req.$target.removeClass('impact');
@@ -957,10 +969,6 @@
                 }
             }
 
-            req.$target.data('lastUpdate', (new Date()).getTime());
-            delete this.requests[req.$target.attr('id')];
-            this.icinga.ui.fadeNotificationsAway();
-
             var extraUpdates = req.getResponseHeader('X-Icinga-Extra-Updates');
             if (!! extraUpdates && req.getResponseHeader('X-Icinga-Redirect-Http') !== 'yes') {
                 $.each(extraUpdates.split(','), function (idx, el) {
@@ -1253,8 +1261,6 @@
                 }
             }
 
-            $container.trigger('beforerender', [content, action, autorefresh, scripted, autoSubmit]);
-
             var discard = false;
             $.each(_this.icinga.behaviors, function(name, behavior) {
                 if (behavior.renderHook) {
@@ -1272,6 +1278,8 @@
             });
 
             if (! discard) {
+                $container.trigger('beforerender', [content, action, autorefresh, scripted, autoSubmit]);
+
                 if ($container.closest('.dashboard').length) {
                     var title = $('h1', $container).first().detach();
                     $container.html(title).append(content);
@@ -1337,6 +1345,8 @@
 
             // Re-enable all click events (disabled as of performance reasons)
             // $('*').off('click');
+
+            return ! discard;
         },
 
         /**