Regression: vertical tabs are not keyboard accessible.

Bumping to major. This is a failure at WCAG level A, and a regression which WILL break Drupal for a clear group of users:

• Definitely fails SC 2.1.1 (keyboard operable)
• Borderline failure of SC 2.1.2 (no-keyboard-trap). Technically, you can get out of it by pressing enter, then tabbing past the options in that field. It's a bit of a WTF barrier until you work that out though.

I've been looking at the D7 and D8 versions of vertical-tabs.js, and I have a fix. Expect a patch soon.

This might turn out to be an easy fix.

Explanation: the patch in #4 nests event.preventDefault() correctly, so we only prevent default link behaviour when the enter key is pressed. Pressing tab, page up, space, etc. should continue to have the default behaviour.

Here is what D7 does. We only prevent default behaviour when the enter key is detected:

  // Keyboard events added:
  // Pressing the Enter key will open the tab pane.
  this.link.keydown(function(event) {
    if (event.keyCode == 13) {
      self.focus();
      // Set focus on the first input field of the visible fieldset/tab pane.
      $("fieldset.vertical-tabs-pane :input:visible:enabled:first").focus();
      return false;
    }
  });

Compare this with D8. Default behaviour is prevented regardless of which key was pressed. Now, the enter key is the only key which does anything:

    // Keyboard events added:
    // Pressing the Enter key will open the tab pane.
    this.link.on('keydown', function (event) {
      event.preventDefault();
      if (event.keyCode === 13) {
        self.focus();
        // Set focus on the first input field of the visible details/tab pane.
        $(".vertical-tabs__pane :input:visible:enabled").eq(0).trigger('focus');
      }
    });

Consequently, the user cannot press the tab key to move focus to the next vertical tab link.

The patch in #4 corrects this by nesting event.preventDefault() correctly.

Thanks googletorp, but we're still waiting for the test bots to smile ;-)

Awesome, thanks googletorp.