When the first two elements in a scrollspy content block have a document
offset of zero (i.e. they're hard against the top of the page),
Scrollspy would switch between them on every scroll event.

This could happen, for example, in a system of nested sections:

```
<section id="animals">
  <section id="dogs">
	Content
  </section>
</section>
```

This ocurred because Scrollspy's check to see if it's at the end of the
array of sections uses `!arr[index]`. This misses the case where
`arr[index]` does exist and is zero.

This commit explicitly checks the array bounds.

Since only on iOS tests fail when the fixture is displayed off-screen, see https://travis-ci.org/twbs/bootstrap/jobs/52605742

Fixes #15741
Closes #15751 by merging it.

Refs http://qunitjs.com/upgrade-guide-2.x/#replace-qunit-jsdump-with-qunit-dump

Refs http://qunitjs.com/upgrade-guide-2.x/#replace-qunit-log-callback-with-qunit-log-callback-for-all-reporting-callbacks

Refs #15893

Fixed performance bug where a tooltip element is created and then immediately destroyed when tooltip.hide() is called and there is no existing tooltip element

Resolves #15557 by merging it.

Fixes #15147.
Closes #15520 by merging it.

[Ref #15881] Use Explicit Values for javascript variables rather than chained ones.

From https://github.com/twbs/bootstrap/pull/15881#discussion_r25330647

Redo of #15876

Add tests for `aria-hidden` of modal

Use strictEqual of 1 here instead of 2 asserts

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Popover is extend Tooltip, the tip function can use tootltip's tip, so we can remove the tip function definition in popover to reduce the js filesize

tip function definition in tooltip.js https://github.com/tbs/bootstrap/blob/master/js/tooltip.js#L405-L407

isHorizontal => isVertical

Refs #15186

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Closes #15484

This reverts commit 1b323762.
This reverts PR #14189 because it caused major regressions.

Fixes #15168.

We'll try to revisit #14167's feature request in Bootstrap v4.

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Special thanks to @programcsharp

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New rule added in JSCS v1.9.0
Our JS already complies with this.

Fixes #15353.