diff --git a/lib/outputrenderers.php b/lib/outputrenderers.php
index d7399fd54fe..5e60efd74fb 100644
--- a/lib/outputrenderers.php
+++ b/lib/outputrenderers.php
@@ -946,6 +946,39 @@ class core_renderer extends renderer_base {
         return $output;
     }
 
+    /**
+     * The standard HTML that should be output just before the <footer> tag.
+     * Designed to be called in theme layout.php files.
+     *
+     * @return string HTML fragment.
+     */
+    public function standard_after_main_region_html() {
+        global $CFG;
+        $output = '';
+        if ($this->page->pagelayout !== 'embedded' && !empty($CFG->additionalhtmlbottomofbody)) {
+            $output .= "\n".$CFG->additionalhtmlbottomofbody;
+        }
+
+        // Give subsystems an opportunity to inject extra html content. The callback
+        // must always return a string containing valid html.
+        foreach (\core_component::get_core_subsystems() as $name => $path) {
+            if ($path) {
+                $output .= component_callback($name, 'standard_after_main_region_html', [], '');
+            }
+        }
+
+        // Give plugins an opportunity to inject extra html content. The callback
+        // must always return a string containing valid html.
+        $pluginswithfunction = get_plugins_with_function('standard_after_main_region_html', 'lib.php');
+        foreach ($pluginswithfunction as $plugins) {
+            foreach ($plugins as $function) {
+                $output .= $function();
+            }
+        }
+
+        return $output;
+    }
+
     /**
      * Return the standard string that says whether you are logged in (and switched
      * roles/logged in as another user).
diff --git a/message/lib.php b/message/lib.php
index 5b22e8bef05..87c2680d2c2 100644
--- a/message/lib.php
+++ b/message/lib.php
@@ -808,7 +808,7 @@ function core_message_render_navbar_output(\renderer_base $renderer) {
  *
  * @return string HTML
  */
-function core_message_before_standard_top_of_body_html() {
+function core_message_standard_after_main_region_html() {
     global $USER, $CFG, $PAGE;
 
     // Early bail out conditions.
diff --git a/theme/boost/scss/moodle/message.scss b/theme/boost/scss/moodle/message.scss
index 0f173587068..9afd874b9b8 100644
--- a/theme/boost/scss/moodle/message.scss
+++ b/theme/boost/scss/moodle/message.scss
@@ -1200,7 +1200,7 @@ $message-drawer-width: 320px;
     right: 0;
     height: calc(100% - #{$navbar-height});
     width: $message-drawer-width;
-    z-index: $zindex-fixed;
+    z-index: $zindex-sticky;
     box-shadow: -2px 2px 4px rgba(0, 0, 0, .08);
     display: flex;
     flex-direction: column;
diff --git a/theme/boost/style/moodle.css b/theme/boost/style/moodle.css
index 29a506c8c34..58768bc7ca1 100644
--- a/theme/boost/style/moodle.css
+++ b/theme/boost/style/moodle.css
@@ -14077,7 +14077,7 @@ a.ygtvspacer:hover {
   right: 0;
   height: calc(100% - 50px);
   width: 320px;
-  z-index: 1030;
+  z-index: 1020;
   box-shadow: -2px 2px 4px rgba(0, 0, 0, 0.08);
   display: flex;
   flex-direction: column;
diff --git a/theme/boost/templates/columns1.mustache b/theme/boost/templates/columns1.mustache
index d2c11db97f5..022df603aa6 100644
--- a/theme/boost/templates/columns1.mustache
+++ b/theme/boost/templates/columns1.mustache
@@ -56,6 +56,9 @@
             </div>
         </div>
     </div>
+
+    {{{ output.standard_after_main_region_html }}}
+
 </div>
 {{{ output.standard_end_of_body_html }}}
 </body>
diff --git a/theme/boost/templates/columns2.mustache b/theme/boost/templates/columns2.mustache
index 70470552193..7b329cbecfc 100644
--- a/theme/boost/templates/columns2.mustache
+++ b/theme/boost/templates/columns2.mustache
@@ -87,6 +87,7 @@
             </div>
         </div>
     </div>
+    {{{ output.standard_after_main_region_html }}}
     {{> theme_boost/nav-drawer }}
     {{> theme_boost/footer }}
 </div>
diff --git a/theme/bootstrapbase/layout/columns1.php b/theme/bootstrapbase/layout/columns1.php
index 59afcc63944..a43de787c86 100644
--- a/theme/bootstrapbase/layout/columns1.php
+++ b/theme/bootstrapbase/layout/columns1.php
@@ -1,4 +1,5 @@
 <?php
+
 // This file is part of Moodle - http://moodle.org/
 //
 // Moodle is free software: you can redistribute it and/or modify
@@ -69,6 +70,8 @@ echo $OUTPUT->doctype() ?>
         </section>
     </div>
 
+    <?php echo $OUTPUT->standard_after_main_region_html() ?>
+
     <footer id="page-footer">
         <div id="course-footer"><?php echo $OUTPUT->course_footer(); ?></div>
         <p class="helplink"><?php echo $OUTPUT->page_doc_link(); ?></p>
diff --git a/theme/bootstrapbase/layout/columns2.php b/theme/bootstrapbase/layout/columns2.php
index 344c2672520..bdd9b85414f 100644
--- a/theme/bootstrapbase/layout/columns2.php
+++ b/theme/bootstrapbase/layout/columns2.php
@@ -69,6 +69,7 @@ echo $OUTPUT->doctype() ?>
         </section>
         <?php echo $OUTPUT->blocks('side-pre', $sidepre); ?>
     </div>
+    <?php echo $OUTPUT->standard_after_main_region_html() ?>
 
     <footer id="page-footer">
         <div id="course-footer"><?php echo $OUTPUT->course_footer(); ?></div>
diff --git a/theme/bootstrapbase/layout/columns3.php b/theme/bootstrapbase/layout/columns3.php
index f9635daa94c..0689a3e70c5 100644
--- a/theme/bootstrapbase/layout/columns3.php
+++ b/theme/bootstrapbase/layout/columns3.php
@@ -78,6 +78,7 @@ echo $OUTPUT->doctype() ?>
         </div>
         <?php echo $OUTPUT->blocks('side-post', $sidepost); ?>
     </div>
+    <?php echo $OUTPUT->standard_after_main_region_html() ?>
 
     <footer id="page-footer">
         <div id="course-footer"><?php echo $OUTPUT->course_footer(); ?></div>
diff --git a/theme/clean/layout/columns1.php b/theme/clean/layout/columns1.php
index 53042bcd739..afa8cc1c2ce 100644
--- a/theme/clean/layout/columns1.php
+++ b/theme/clean/layout/columns1.php
@@ -70,6 +70,7 @@ echo $OUTPUT->doctype() ?>
             ?>
         </section>
     </div>
+    <?php echo $OUTPUT->standard_after_main_region_html() ?>
 
     <footer id="page-footer">
         <div id="course-footer"><?php echo $OUTPUT->course_footer(); ?></div>
diff --git a/theme/clean/layout/columns2.php b/theme/clean/layout/columns2.php
index 66c8c9e8cd8..e233a73ee82 100644
--- a/theme/clean/layout/columns2.php
+++ b/theme/clean/layout/columns2.php
@@ -72,6 +72,7 @@ echo $OUTPUT->doctype() ?>
         <?php echo $OUTPUT->blocks('side-pre', $sidepre);
         ?>
     </div>
+    <?php echo $OUTPUT->standard_after_main_region_html() ?>
 
     <footer id="page-footer">
         <div id="course-footer"><?php echo $OUTPUT->course_footer(); ?></div>
diff --git a/theme/clean/layout/columns3.php b/theme/clean/layout/columns3.php
index 44bc41fdc30..a126137f667 100644
--- a/theme/clean/layout/columns3.php
+++ b/theme/clean/layout/columns3.php
@@ -85,6 +85,7 @@ echo $OUTPUT->doctype() ?>
         </div>
         <?php echo $OUTPUT->blocks('side-post', $sidepost); ?>
     </div>
+    <?php echo $OUTPUT->standard_after_main_region_html() ?>
 
     <footer id="page-footer">
         <div id="course-footer"><?php echo $OUTPUT->course_footer(); ?></div>