The behavior of OO.ui.FieldLayout is inconsistent (and sometimes broken) when the label contains HTML links (<a href> tags), depending on the widget used in the FieldLayout.
When the widget returns a value from getInputId(), FieldLayout behaves consistently with the HTML <label> element: clicking text focuses the widget, while clicking a link activates the link without focusing the widget.
When the widget returns null from getInputId(), however, FieldLayout always (tries to) focus the widget and never follows the link.
This regression seems to have been introduced in rGOJU383d0c89477e: Allow more widgets to be focussed programatically, although it didn't affect ApiSandbox as originally reported until rGOJUea1f002af2fe: Re-introduce .simulateLabelClick() as a separate method from .focus() due to the lack of a focus() method on OO.ui.Widget.
Demo:
new OO.ui.FieldLayout( new OO.ui.TextInputWidget( {} ), { label: new OO.ui.HtmlSnippet( '<a href="http://example.com">This link</a> works' ), align: 'top' } ), new OO.ui.FieldLayout( new OO.ui.ButtonWidget( { label: 'Button' } ), { label: new OO.ui.HtmlSnippet( '<a href="http://example.com">This link</a> does not work' ), align: 'top' } ),
Original report:
All links in the container of the ApiSandbox don't work. You have to use the middle mouse button or select the option to open links in a new tab / window to reach the links. The links at the top of this special page do open in a new tab / window. Different browsers (Edge, Firefox, Chrome) and different wikis (nlwiki, metawiki, wikidatawiki) give the same result, so it's not specific to any browser or wiki.
I don't know if this was done for security purposes, but if it is, you can close this ticket as invalid.