Cursor position is inconsistent after Page Up or Page Down

[peterflynn]

1. Open a longish file (I used BootstrapReporter.js with window height set to show about 72 1/2 lines).
2. Page Down a few times
3. Go back up to the top and put the cursor on the first line
4. Scroll down just a few pixels (hiding all the padding above line 1 but leaving line 1 itself at least partially visible)
5. Page Down a few times

Result:
step 2 - after each Page Down, cursor is on second visible line (not topmost)
step 5 - after each Page Down, cursor is on first (topmost) visible line

You can repro the same issue by starting at the bottom and hitting Page Up.

Note: it's currently easy to hit this when using Ctrl+Home/End (Cmd+Up/Down on Mac) due to #1024, which is essentially a shortcut do doing steps 3-4. But you can also hit this by manually adjusting the scroll position.

[peterflynn]

Page Up/Down scrolling behavior was much worse before the recent CodeMirror merge (see #159), so I'm hesitant to call this a regression... I don't think we should try to fix this during Sprint 10.

[pthiess]

Reviewed -

[njx]

No longer repros for me with cmv3 merged into master (although we still have the other issue of the first page down). FBNC @peterflynn

[peterflynn]

The cursor position is now consistent, though it's unfortunately consistently in completely the wrong location (bottom of the viewport instead of top like all other editors -- see side note in #2810). This older bug seems closable, though.