Don't allow tabbing to elements inside hidden parents

[benadamstyles]

What: Don't allow tabbing to elements inside hidden parents.

Why: I think this correctly matches browser behaviour.

How: Add a !el.closest('[hidden]') check in the focusable elements filter.

Checklist:

• Documentation N/A
• Tests
• Typings N/A
• Ready to be merged
  

[codecov]

Codecov Report

Merging #556 (298cd9f) into master (217b487) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #556   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           13        13           
  Lines          696       698    +2     
  Branches       218       220    +2     
=========================================
+ Hits           696       698    +2     

Impacted Files	Coverage Δ
src/tab.js	100.00% <100.00%> (ø)

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[ph-fritsche]

Thanks for the report. Yes, we should exclude hidden elements. :)

But while elements with hidden attribute should not contain elements that are relevant for the user, they are still displayed and available if a stylesheet overwrites display.

<input/>
<button hidden style="display: inline-block">click</button>
<input />

So we need to traverse the ancestors and verify if they are visible.

I've opened benadamstyles#1 with a solution.
But the bug in jsdom causing window.getComputedStyle() to ignore hidden attributes has to be resolved first.

[ph-fritsche]

Replaced by #579