bootstrap 3.4.1 regression (https://github.com/opnsense/core/issues/3377), since our modals aren't placed directly under the body tag, the backdrop seems to be disfunctional. https://github.com/opnsense/core/issues/3377

Although its probably not the best fix, it does the trick.

src/opnsense/mvc/app/views/OPNsense/CaptivePortal/index.volt

@@ -247,6 +247,7 @@ POSSIBILITY OF SUCH DAMAGE.
 
 <!-- upload (new) template content dialog -->
 <div class="modal fade" id="DialogTemplate" tabindex="-1" role="dialog" aria-labelledby="formDialogTemplateLabel" aria-hidden="true">
+    <div class="modal-backdrop fade in"></div>
     <div class="modal-dialog modal-lg">
         <div class="modal-content">
             <div class="modal-header">

src/opnsense/mvc/app/views/OPNsense/Firewall/alias.volt

@@ -397,6 +397,7 @@
 
 {# Edit dialog #}
 <div class="modal fade" id="DialogAlias" tabindex="-1" role="dialog" aria-labelledby="DialogAliasLabel" aria-hidden="true">
+    <div class="modal-backdrop fade in"></div>
     <div class="modal-dialog modal-lg">
         <div class="modal-content">
             <div class="modal-header">

src/opnsense/mvc/app/views/layout_partials/base_dialog.volt

@@ -57,6 +57,7 @@ label           :   dialog label
 {% endfor %}
 
 <div class="modal fade" id="{{base_dialog_id}}" tabindex="-1" role="dialog" aria-labelledby="{{base_dialog_id}}Label" aria-hidden="true">
+    <div class="modal-backdrop fade in"></div>
     <div class="modal-dialog modal-lg">
         <div class="modal-content">
             <div class="modal-header">

src/opnsense/mvc/app/views/layout_partials/base_dialog_processing.volt

@@ -1,5 +1,6 @@
 <!-- Static Modal "Processing request", prevent user input while busy -->
 <div class="modal modal-static fade" id="processing-dialog" role="dialog" aria-hidden="true">
+    <div class="modal-backdrop fade in"></div>
     <div class="modal-dialog">
         <div class="modal-content">
             <div class="modal-body">

src/opnsense/mvc/app/views/layouts/default.volt

@@ -285,6 +285,7 @@
 
     <!-- dialog "wait for (service) action" -->
     <div class="modal fade" id="OPNsenseStdWaitDialog" tabindex="-1" data-backdrop="static" data-keyboard="false">
+      <div class="modal-backdrop fade in"></div>
       <div class="modal-dialog">
         <div class="modal-content">
           <div class="modal-body">