Imac dims unexpectedly

Question

having the odd issue with my late 2012 27' 3.2ghz imac
not regularly but now and then whilst having a few applications open, i find whilst moving the cursor (maybe always in a downward motion - if i remember correct) the screen can dim and a few other things happen
tonioght the screen dimmed, all open applications/pages disappeared off screen and the screen darkened (NOT totally) except for the dock.
a few months ago, a few times one night the screen darkened with a broken line appearing a few inches above the dock - i think a few pages/applications may have dropped under this line - i cant remember too much about what happened these times but the screen definitely darkened, the line appeared and i THINK some apps/screens/pages went under the line - the last bit i'm not 100% sure
this all has only happened 5 or 6 times in 3 months.
i then click on the screen and everything comes back to where it was
i have not touched expose serttings - for what that's worth
when i mentioned it to someone at the genius bar when i took the imac in for pixel issues it was brushed off in a "no idea what that could be"
should i be concerned?
what might this be?
if it happens again how best to take note of this to have apple look at it with an idea of what might be happeneing?
any views / knowledge / experience wquth this issue
cheers
peter t

Answer 1

also
i have a mouse and trackpad in operation with my imac - could something be at play between these two.
the issue happened tonight whilst using the mouse after clicking and closing an open page and moving the mouse down
i cant remember if previous occurrences occurred using trackpad or mouse - sorry
any educated ideas /answers appreciated
****** update ******
in playing around with this issue and the trackpad and mouse
i'm thinking that maybe what has been haooening is the equivalent to 4 finger downwrd motion on the trackpad -= i'm happy to accept that that might have been previous events and leave that there for now HOWEVER......
the question i have for tonight's shananingans (ie all darkens except the dock) is can the 4 finger downward motion on the trackpad be repicated on the mouse - as that wasa what i was using\\this may be a simple answer and that a relative novice (5 months) is not aware of
thanks again