Fix vertical content jumps in the BlockNote editor on selection

BlockNote 0.51 puts the className we pass to <BlockNoteView>
(`block-note-editor-container`) onto BOTH the outer `.bn-container`
wrapper AND an inner wrapper that does NOT carry `.bn-container`. With
the previous selector matching by class name alone, every rule cascaded
onto both nesting levels — most importantly `display: flex`,
`flex-direction: column-reverse` and `gap: 10px`.

Two flex layouts stacked one inside the other meant that whenever the
side menu / drag handle plugin views re-rendered (which happens every
time the selection moves or the mouse leaves the editor), both layout
calcs ran and the inner wrapper's gap shifted the visible content by a
few pixels.

Tightening the selector to `.block-note-editor-container.bn-container`
restricts the rules to the outer wrapper only; the inner wrapper falls
back to defaults (`display: block`, no gap) and stops contributing to
the layout.

Refs https://community.openproject.org/wp/STC-779

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

modules/documents/app/assets/stylesheets/_index.sass

@@ -4,7 +4,14 @@ $blocknote-max-width: 800px
   .ck-content
     min-height: 30vh
 
-.block-note-editor-container
+// BlockNote 0.51 stamps `.block-note-editor-container` (the className we
+// pass to <BlockNoteView>) onto BOTH its outer `.bn-container` wrapper AND
+// an inner wrapper without `.bn-container`. Without the `.bn-container`
+// requirement, the rules below cascade onto both elements — `gap: 10px`
+// and `flex-direction: column-reverse` get applied at two nesting levels,
+// which shows up as a few-pixel vertical layout jump whenever the side
+// menu / drag handle re-renders during selection.
+.block-note-editor-container.bn-container
   align-items: center
   position: relative
   display: flex