In the layers dialog, there are two things you can lock and unlock: pixels (looks like a paintbrush icon), and the alpha channel (looks like a checkerboard next to the pixels icon).
Supposedly, you click the alpha channel to lock it, and then you can't change the alpha channel. When you click it again, it's supposed to be unlocked and you can change it.
The icons seem to vaguely change from locked (looks like a fat checkerboard, or one with rounded corners) to unlocked (sharp, square corners) - all somewhat visible when you change the focus to a different control on the screen.
The designer of this concept should be put in front of a firing squad, but now that I know how to lock AND UNLOCK, the unlock seems not to work: I can't change the color to alpha to make a transparent background. I'd been doing it all day with other images; now I can no longer do this operation, even with images I'd previously changed.
Am I missing a global setting somewhere? I just need to change a color to alpha.
I'm using Gimp 2.8.14
Here are screenshots:
It's hard to tell, but the top one's lock icon is a little fatter and rounded in the corners than the bottom one. Those are the locked and unlocked modes for the alpha channel.
I've restarted Gimp, and reloaded previous images which I had no problems editing.