You have two choices. One is to drag the calendar tab on top of Notes or any other Tool, then click whichever tool you wish to be on top, so that the Calendar hides behind it and only the tab is visible. In fact you can hide all the other Tools (Notes, Properties, Reports, etc.) behind any one of them.
The second is that if you need a lot of room for the plex (which you call the Brain network), double-click in the plex and all the Notes will be hidden. Likewise, if you want to fill the screen with Notes (or any other Tool) and hide the plex, double-click the tab. Double-click again to change it back to normal.