[visible.directive] Set visible to false only when fully hidden. Fix #19

[artoale]

I've tried to fix #19 by checking if fully visible/hidden before setting the value.

This has two "issues" I could think of:

• Inconsistency: the binded variable becomes "true" when it's fully visible, but stays as that untill is fully hidden. That's the desired behavior, but I'm just worried it might somehow lead to confusion
• Impossible to "clear" when there's not enough room for completely hiding the target. This also is not necessarily an issue, and it's a drawback of our decision. (We would have it anyway, even with the previous solution, if the case the page is not big enough to hide the target even partially).

I believe those are not actual issues - but might be worth discuss it/keep track of it

[artoale]

@flea89 pinging you in for review.

[artoale]

Also, we should definitely add some tests for this scenarios (even simple ones that skips the body/scroller element complexity)

[flea89]

Nice catch. LGTM.
Rebasing on develop and merging.

I totally agree with you about the tests. The API are quite covered while the scroll functionality is not.
I reckon this might require some discussion though. I haven't found a nice way to test the internals so far, we can explores different way to do it though and see whatever works best.