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Fixes #18493
Closes #18836
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Add documentation to components/utilities to publicly expose the
.pos-f-t utility class to users.

Closes #18799
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It's very handy to add another spacer size (e.g. `4`) which can easily be done by extending the map.
Example:
```
$spacers: (
  0: (
    x:   0,
    y:   0
  ),
  1: (
    x:   $spacer-x,
    y:   $spacer-y
  ),
  2: (
    x:   ($spacer-x * 1.5),
    y:   ($spacer-y * 1.5)
  ),
  3: (
    x:   ($spacer-x * 3),
    y:   ($spacer-y * 3)
  ),
  4: (
    x:   ($spacer-x * 5),
    y:   ($spacer-y * 5)
  )
);
```

Closes #18706
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Ports #18872 to v4
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Fixes #17584
Closes #17607 by merging it

- Add/replace callout for color and AT with include

This one was overlooked at edit

See #17719 and #19199

http://bugzil.la/1060131 has been fixed by http://bugzil.la/823483 which should land in Firefox 46.
Refs #14320
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http://bugzil.la/975632 has been fixed by http://bugzil.la/823483 which should land in Firefox 46.
Refs #10690
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Alternate fix to #17965: Updates to use the new class name and adds some basic docs for custom checkbox/radio validation

Fixup for fb4f5f0b
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Fixup for 365db6e3
/fyi @mdo
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Wasn't able to reproduce in iOS 9.2.
So remove its entry from the Wall of Browser Bugs.
Refs #14708
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Fixup for 27ba4beb
/fyi @mdo
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