Round return values of getMaxScrollTop, getScrollHeight

[nathansobo]

Fixes #15343

If the current scrollTop/scrollLeft value exceeds the maxScrollTop / maxScrollLeft, we reassign the values to the max on read. This allows us to behave correctly when the editor has changed in dimensions or the content has changed since the scroll position was previously assigned.

Prior to this PR, we were applying Math.round to the scroll positions when assigning them but not rounding the values when reassigning them to the max.

In situations where the content height contains fractional pixels, this can lead to a fractional-pixel scroll position when scrolled all the way down/right which causes blurry text. This can also cause unexpected firing of onDidChangeScrollTop when the value isn't actually changing due to a decision to notify listeners based on a rounded-up value not matching a fractional clipped value, as was the case in #15343.

/cc @t9md @Ben3eeE

[nathansobo]

@t9md Could you give this a try to make sure it fixes your infinite loop issue?

[t9md]

@nathansobo Confirmed fix with

• reproduction procedure described in onDidChangeScrollTop fired when no scrollTop change / can cause infinite loop #15343
• original issue in my atom-narrow package.

No infinite loop for both now, thanks!

Tested with latest build on cd82330 commit.

[130] ~/Downloads% ./Atom\ 8.app/Contents/MacOS/Atom  --version
Atom    : 1.21.0-dev-cd82330
Electron: 1.6.9
Chrome  : 56.0.2924.87
Node    : 7.4.0