01-06-2019, 08:50 PM
I do not know about the Mac version of Gimp but the description sounds like the Gimp 2.10 Unified Transform tool. To constrain the proportions try:
Click in the large corner square, it should have changed colour and the icon to a 'scaling type'
Hold the ctrl key down (it will be command in OSX I suppose) and drag.
Click in the large corner square, it should have changed colour and the icon to a 'scaling type'
Hold the ctrl key down (it will be command in OSX I suppose) and drag.