MDL-48865 core_block: Show regions with only hidden blocks when editing

Before this patch if all the blocks in a block region are set to hidden the region will be docked even when editing is enabled.
This meant that as a user I would need to add a new block to the page and move it to that region via the configuration
menu before I could manipulate the hidden blocks again.

lib/blocklib.php

@@ -517,6 +517,11 @@ class block_manager {
             return false;
         }
 
+        // Block regions should not be docked during editing when all the blocks are hidden.
+        if ($this->page->user_is_editing() && $this->page->user_can_edit_blocks()) {
+            return false;
+        }
+
         $this->check_is_loaded();
         $this->ensure_content_created($region, $output);
         if (!$this->region_has_content($region, $output)) {