[#53451] make editor toolbar sticky in mobile view

This was not working due to the overflow attribute being set to
something else than visible or clip. As `clip` is mostly the same as
`hidden`, I was able to restore the stickiness by using that.

See
https://benfrain.com/yes-you-can-use-position-sticky-and-overflow-together/

frontend/src/global_styles/layout/work_packages/_full_view.sass

@@ -48,7 +48,7 @@
   &--split-left
     border-right: 1px solid var(--borderColor-default)
     overflow-y: auto
-    overflow-x: hidden
+    overflow-x: clip
     flex: 2
     position: relative
     @include styled-scroll-bar

frontend/src/global_styles/layout/work_packages/_mobile.sass

@@ -50,6 +50,13 @@
           margin: 0
           width: initial
           border: none
+          // Set overflow to clip so that the editor toolbar can stick, see WP#53451
+          overflow-y: clip
+
+          // We need to cancel out the padding of the page content.
+          // Otherwise, the scrolled content will be visible above the toolbar
+          .document-editor__toolbar
+            top: -10px
 
           .work-packages--panel-inner
             padding: 5px 0 20px 0