electronics made easy
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Open the example LiquidCrystalDisplay.fz (any two-layer design would work)
Switch to PCB view.
Select the PCB.
Press Delete key. (PCB is now deleted.)
Press Ctrl-Z or use Undo menu item.
Result: PCB is restored, but it is now single layer and all top copper wires are lost. The Inspector still reports PCB to be double layer but this is not true.
[Note that this can happen by mistake. The only way to select vias is to draw a selection box from outside. This always selects the PCB. If one forgets to deselect the PCB, then there is risk to delete it PCB by mistake while deleting a via. It would be nice to be able to lock the PCB so that it cannot be deleted by mistake. Assuming there are good reasons to delete one's PCB, I think this is at most a very occasional task in a project life, so an additional safety could be added.]
Fritzing 0.6.5 (b5717 2012-01-02) [Qt 4.7.4]
Confirming the bug--sorry about that. There is a way to lock a part (under the Part menu), but unfortunately it only blocks move and rotate, not deletion. Adding both of these to the issue tracker.
Both issues will be fixed in the next release: a locked part will block deletion, and undoing a deleted 2-sided pcb will restore the two sides.