Keep last image in memory

[marcbouchenoire]

Instead of switching to an empty state when you change your selection to a layer that doesn't contain bitmap data, it should keep the last image (Makes copy-pasting color values to other layers easier and smoother).

[marcbouchenoire]

Experimentation that shows the empty state on launch only and then keeps the last image available (and its colors) when changing selection to nothing or a layer with no bitmap data. It might need some polishment to make it clear that it's a previous image.