PageUp can cause jumpiness in editor scrolling

[nacin]

Initially reported by matt, and the two of us were the only people who could reproduce. (See IRC discussion to be linked.) Video: ​https://cloudup.com/igMKu-5ANFd.

Turns out our mceKeyup listener doesn't bail on pageup/pagedown. It should bail on special keys along the lines of [29535], though the only ones that should actually cause a problem are pageup/pagedown/home/end. Two different patches attached depending on the level of coverage we need.

[nacin]

Pretty sure this is all that is needed.

[iseulde]

What about fn + arrow keys? fn is not detectable...

[iseulde]

Nevermind, key codes change...

[iseulde]

For me fn + up = 38, fn + down = 40.

[nacin]

Okay, now I understand how this is reproduced. Scroll down a post, click somewhere, then scroll up far enough until the cursor is out of the screen. Then start hitting special keys that *shouldn't* cause it to scroll back to the cursor.

This isn't just pagedown/pageup. It's also all modifier keys, including Windows/Mac (cmd) keys, function keys, etc... Basically, everything in [29535], just not being DRY about it.

29485.3.diff​

[nacin]

In 29692:

Editor scrolling: Avoid jumping to the cursor when modifier and scrolling keys are pressed.

fixes #29485.