Two bugs/issues I’ve found, and wanted to let you guys know about if they hadn’t already been mentioned!
When I add an icon to a mockup, it has a dark grey colour by default. If I want to set the colour of the icon to black, I select the colour option and choose black, but nothing happens. I have to change the colour to something other than black, and only then can I change it back to black. It seems like Mockups is assuming the default colour of the icon is black, and is not letting me set that as a colour.
The keyboard shortcut for nudge-resizing an item is also the default shortcut on my computer for changing the orientation of my display. When I try to nudge resize an item, instead my screen/displays flips to portrait mode! I can still resize using the controls in the top right, and I’m sure I could change the shortcut settings in my graphics control panel, but I just wanted to point this out since it’s likely to be the default shortcut on many other computers as well - at least in Windows.