I find my self constantly clicking the top navigation bar when I have an item open for editing. The current default behavior is that the top menu is rendered inactive in such a program state.
From a system state it make sense to require that a user not leave without some determination of whether they want the item saved or cancelled. The frequency that I make this mistake even though I know that the system blocks this would suggest that it is not a usable convention.
A more user friendly approach may be to leave the top menu active, but add a pop-up that either saves, cancels any changes and continues navigation, or cancels navigation away from the page. Cancel any changes and continue navigation would make sense as the preselected choice when just enter is selected.