Why does Ctrl+ScrollLock cancel dialogs?
Commenter Adam Russell asks
why Ctrl+ScrollLock cancels dialogs.
Easy.
Because Ctrl+ScrollLock is the same as Ctrl+Break,
and Ctrl+Break cancels dialogs.
Okay, that answer actually raises two more questions.
First, why is Ctrl+ScrollLock the same as Ctrl+Break?
This is a consequence of the backward compatibility designed
into the Enhanced Keyboard layout which is in widespread use today.
If you go back and look at the original
PC/XT keyboard layout,
you'll see that many of the keys we're familiar with today
simply didn't exist back then,
and one key (PrtSc) existed in a very different form.
Let's start with PrtSc.
Observe that it was originally a shifted key, atop the asterisk.
In the
PC/AT keyboard layout,
the PrtSc/* key migrated into the numeric keypad,
and with the introduction of the
enhanced keyboard layout,
the two functions PrtSc and multiplication were split into
two separate keys.
The asterisk stayed with the numeric keypad,
while the PrtSc function moved to the top row as a function key
(sharing a key with the SysRq key).
Okay, that's all nice, but what about Ctrl+ScrollLock and
Ctrl+Break?
Well, in the original PC/XT keyboard layout, there was no
Break key.
The key sequence for Break was Ctrl+ScrollLock.
(And for completeness, the key sequence for Pause was
Ctrl+NumLock.)
Even though the enhanced keyboard moved
the Pause and Break functions to their own key,
pressing the Pause key internally generated scan codes
that simulated a press of Ctrl+NumLock.
In other words, when you pressed Pause, the keyboard hardware
actually tells the computer,
"The user pressed the Ctrl key and then pressed the NumLock key."
Similarly, when you pressed Ctrl+Break, the keyboard hardware
tells the computer,
"The user pressed the Ctrl key and then pressed the ScrollLock key."
Therefore, Ctrl+ScrollLock acts like Ctrl+Break because at the hardware
level, they are the same thing.
That the two functions exist on separate keys is just a fake-out
by the keyboard hardware.
Okay, so now that we understand why Ctrl+ScrollLock is the same
as Ctrl+Break, the next question is why Ctrl+Break cancels a dialog box.
If you look at the list of virtual key codes in winuser.h,
you'll find lots of virtual keys that don't exist on the PC keyboard:
VK_CLEAR,
function keys VK_F13 through VK_F24,
and the long-forgotten VK_PA1.
What are all these things?
In the beginning, Windows and MS-DOS ran on more than just the IBM PC.
I know for certain that it also ran on the NEC PC-98,
and there were probably other architectures that were either explicitly
supported or for which support was planned or reserved.
The designers of the original Windows keyboard input driver model wanted to
cover all of the likely bases and included support for keys beyond the
basic 84-key PC/XT keyboard,
keys that could be found on then-popular keyboard layouts such as the
VT-100 and even the 3270.
I don't know which keyboard had a Cancel key,
but presumably one did, or at least the people who designed the
input driver model wanted to be prepared for the possibility of one
showing up.
Given that you had a Cancel key,
it seemed natural for the dialog manager to support it by
treating a press of the Cancel key as the same as clicking the
Cancel button on a dialog.
