Modal: position backdrop absolute instead of fixed to..

a) improve the scroll performance
b) prevent that touch/mousewheel scrolling doesn’t work in some cases
c) prevent that scrolling using the scrollbar closes the modal

app/assets/javascripts/app/lib/bootstrap/modal.js

@@ -9,7 +9,6 @@
  /*
   modified by Felix Jan-2014
   - add this.$body = $(options.container || document.body)
-  - adjustBackdrop: also adopt left, top and width from $body
   modified by Felix Jul-2017
   - add rtl support
 */
@@ -248,19 +247,8 @@
 
   Modal.prototype.adjustBackdrop = function () {
     this.$backdrop
-      .css('left', this.$body.offset().left)
-      .css('top', this.$body.offset().top)
-      .css('width', this.$body.width())
       .css('height', 0)
       .css('height', this.$element[0].scrollHeight)
-
-    if(App.i18n.dir() == 'rtl'){
-      this.$backdrop.css('right', 'auto')
-    }
-
-    if(this.scrollbarWidth){
-      this.$backdrop.css('width', this.$body.width() - this.scrollbarWidth)
-    }
   }
 
   Modal.prototype.adjustDialog = function () {

app/assets/stylesheets/zammad.scss

@@ -6486,10 +6486,6 @@ footer {
   max-width: 460px;
 }
 
-.modal-backdrop {
-  position: fixed;
-}
-
 .modal-content {
   border-radius: 0;
   border: 1px solid hsl(0,0%,90%);