When the Project Browser and System Browser are sharing a pane as shown below:

When a project loses focus (to work in another project or a family), then regains focus, the system browser always comes back on top, even if the project browser was on top when the project lost focus.
This is annoying. Please fix it. The application should remember which pane was on top and keep it that way.
Alternate fix idea: have the order of the tabs determine which one is normally on top. Right now, tab order doesn't appear to matter.