Scopes & Mask Tabs disappeared from UV/Viewer screen Any idea how this could have happened?  I have my keyboard connected to 2 computers (via a/b switch) and occasionally type a key sequence into Blender that is meant for the other computer.  I have no idea what I might have typed to remove those tabs and no clue how to get them back.  I'm doing some color correction and really need my scopes back.  BTW, when I open a new Blender session the tabs are missing.  When I open a backup copy of my color correction, the tabs are there (but I would have to trash all my changes to get back to where I left off.)
I figured it out.  I looked in space_image.py and found the following code.  

class ImageScopesPanel:
    @classmethod
    def poll(cls, context):
        sima = context.space_data
        if not (sima and sima.image):
            return False
        # scopes are not updated in paint modes, hide
        if sima.mode == 'PAINT':
            return False
        ob = context.active_object
        if ob and ob.mode in {'TEXTURE_PAINT', 'EDIT'}:
            return False
        return True

I could not quickly figure out the meaning of each of the variables, but decided to take a look at what could turn off the "scopes" tab.  As soon as I turned the "3D View" from "Edit Mode" to "Object Mode", the "Scopes" tab re-appeared on the "UV/Image Editor".