While there are cases when you want to edit the mask straight away (this is
often the case when starting with a white or black mask), in many other cases,
the mask may be already as you want it per its initialization (e.g. when
initializing with a channel, selection, the alpha channel, etc.).
Until now, the Add Mask dialog was always switching to the "Edit Layer Mask"
mode by default, which forced an additional unneeded click each time you created
a mask (and were in a case where you initialize the mask as you want it
directly).
Now adding "Edit mask immediately" feature in the "Add Layer Mask dialog":
* It's checked by default to keep historical behavior.
* As most other dialogs, the last value is remembered, allowing people with
repetitive workflow not to have to repeatedly set the settings each and every
time.
* This default is also visible and settable in Preferences > Interface > Dialog
Defaults.
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