[Dimmer] blurring dimmable leaves z-index stacking side effects

[lubber-de]

Description

Thanks to @mayfield for the Fix (me being a lazy dude kindly copied parts of his issue text 😄 here)

For reference: https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Positioning/Understanding_z_index/The_stacking_context

The use of a filter CSS property will impose z-index stacking rules for an element. As such, when using a blurring modal dimmer there are side effects left over from having a filter property on the non-dimmed content.

This leads to unexpected stacking behavior for elements that would otherwise not have z-index stacking rules applied to them. One example being the use of menus with dropdowns following the open/close of a blurred modal.

Testcase

http://jsfiddle.net/ncoa89yf/2/

• Remove the CSS and open the top dropdown to see the issue
• use the button to proove the transform filter is still animating (because the startfilter is changed in the PR)

Screenshots

Before

After

More screenshots in the original SUI issue below

Closes

Semantic-Org/Semantic-UI#5416

[y0hami]

LGTM

[ColinFrick]

LGTM