Prevent “half screen” bug by resetting scroll position when editor regains focus

[savetheclocktower]

Identify the Bug

See #191.

Description of the Change

Pulsar uses a hidden input element to receive keyboard input in text editors.

When this hidden element is outside the current viewport, sometimes the browser tries to scroll a container that isn't meant to be scrolled in order to bring it into view. This doesn't make sense because of how we handle scrolling, so it's always the wrong thing to do, even though the browser's just trying to be helpful.

Atom used to handle this by explicitly resetting the container's scrollTop and scrollLeft to 0 after the hidden input was focused, but this PR changed that behavior because of a mistaken belief that it wouldn't be needed anymore.

Alternate Designs

You can read about other possible fixes in #191. I think we settled on this one because it's just restoring a fix that Atom used to have, so it's less risky than trying a novel approach that might introduce new regressions.

Possible Drawbacks

I honestly doubt that there are any. If this code caused any problems, it wouldn't have been previously present in Atom.

Verification Process

Perform the reproduction steps in #191 and verify that the issue no longer occurs.

Release Notes

Fixed an issue that sometimes caused text to shift or disappear after an editor pane regains focus.

[bobemoe]

Also fixes #355

[mauricioszabo]

I see absolutely no problem with this, let's get this merged and see if it fixes the issue!

If not, we can always revert - if you want, please add a CHANGELOG too, because this needs to be mentioned - it's such a weird bug and the fix is so... simple... it amazes me!