Alt-PrintScreen doesn't work (!)

[Mark]

I noticed this before, but I didn't pursue it because at the time it
wasn't that important.

I was setting up some screenshots to send in to show what happens for
a particular Chrome bug I've noticed (not all graphics are loaded in
long Google emails with complex graphics) and I noticed rather
pointedly that <alt><print> does nothing.  <print> by itself works
just fine, but (of course) I get the whole screen.  (On my keyboard
the key is labeled Print Screen, but I'm abbreviating here.)

I tried using <shift> and <ctl> and every other combination and no
joy.  I did get an interesting combination of new Seamonkey window,
new Seamonkey composer window based on the new Seamonkey window it
just opened, a bookmark window of some kind that I don't usually see,
and my current (this) window froze with the text up to that point in
the email highlighted.  (This unlocked when I clicked on another
window, minimized it and then had to tell Seamonkey not to close my
right-most tab to which it has somehow shifted the focus, then all was
well again...I think.)

To be on the safe side, and in keeping with some comments I and others
posted recently, I just went into my Keyboard Shortcuts and it said
that my shortcut for "Take a screenshot of a window" was set to
<alt><print>.  I clicked on that and tried to set it for <alt><print>
again, and nothing happened.

I've had issues with the keyboard before, so I thought maybe something
was wrong there (and maybe still is), so I typed <print> instead.
That worked, but it stole the shortcut for "Take a screenshot."  So, I
changed the window screenshot to <ctl><print> and that works.  I then
restored the full screenshot to <print> and that also works.

Note that I tried using both the left and right <alt> keys and neither
one worked.  To rule out a generic hardware problem, I tested other
<alt> key combinations and they seem to be fine, just not this one.

Any ideas (other than "Keep <ctl><print> if that's what works")?

Thanks.