I want to just edit it as a black-and-white image, like any normal layer.
Click the Layer Mask on the Layers Panel, then turn off the visibility of the other channels in the Channels Panel by clicking the eye icon next to the [RGB] (or [CMYK]) channel.
You can then highlight the mask channel and edit it like a greyscale image layer.
(This is really what Option/Alt-clicking the mask thumbnail does)
I found the answer; Alt-clicking the layer mask opens it for editing. (This is seriously awful UI design.)
Answer is correct. Opinion is opinion & others may not agree. FYI, shift-click toggles its visibility too. If you can figure out a simpler method to accessing 3 levels of functionality with a single click, I'm sure Adobe would be happy to hear it.– TetsujinSep 9, 2018 at 9:54
You can edit the layer mask by simply selecting the layer mask thumbnail, and then you can paint on it directly - although by default the layer mask is not visible - it's still fully editable without toggling visibility. Sep 9, 2018 at 11:02
@Tetsujin There should be an ”Edit” option in the context menu of the mask. This is the place people look for options. Alt-clicking is arbitrary and cannot be used unless you know it beforehand. Sep 10, 2018 at 9:16