bug: hiding a node subtree and adding a new node on it by another user leads to broken state

**Describe the bug**
Hiding a node and adding a new node on it by another user leads to a broken state where the user can cycle through groups of nodes instead of hiding / unhiding.

**To Reproduce**
Steps to reproduce the behavior:
1. Start with a couple of nodes
2. User A hides a subtree
3. User B adds a node on this subtree
4. User A shows the subtree which gets in to a broken state

Reproduce the issue using the listed two-user sequence: hide a subtree, add a node to it from the second user, then show the subtree again. Trace the hide/show and node-add synchronization paths to find why the first user cycles through node groups. Done means the subtree can be hidden and shown normally after the concurrent update, without leaving the mindmap in a broken state.

Written by the indexing model from the issue text.