Resolve #3420203 "Bottom section drop zone bad"

Closes #3420203

js/blocks.js

@@ -76,7 +76,7 @@ document.addEventListener('element-dropped', e => {
   if (type === 'section') {
     // Pass either the section ID to put this block in front of, or the string
     // "last" to put it at the end.
-    destination.section = dropZone.nextElementSibling?.classList.contains('lb-plus-section') ? dropZone.nextElementSibling.id : 'last';
+    destination.section = dropZone.getAttribute('preceding-section-id');
   }
   // Check if the block is coming from a different section storage.
   const nestedStoragePathFrom = draggedBlock.closest('[data-nested-storage-path]')?.dataset.nestedStoragePath;
@@ -143,7 +143,7 @@ document.addEventListener('element-dropped', e => {
   if (type === 'section') {
     // Pass either the section ID to put this block in front of, or the string
     // "last" to put it at the end.
-    destination.section = dropZone.nextElementSibling?.classList.contains('lb-plus-section') ? dropZone.nextElementSibling.id : 'last';
+    destination.section = dropZone.getAttribute('preceding-section-id');
   }
 
   // Place the block.