In case anyone found this question via a Google search (as I did), I’ve found an explanation of ‘focusindex’ here: https://forum.mendix.com/link/questions/92170
“As our pages and widgets are dynamic by their nature we need something more flexible then just a flat tab order defined by
tabindex. We use so-called "nested" tab order. Additional custom property
focusindexsimply "marks" a DOM node as a container where
tabindex (or natural tab order) of underlying focus-able elements is "local".
We named it
focusindexto prevent the clash with
tabindex on naturally non focus-able elements. That makes them focus-able and we don't want that in this case.”
Tab index (only in web forms)
The tab index influences the order in which the end user navigates through the form using the tab key. By default tab indices are zero and the tab order is determined automatically by the client system. A value of minus one (-1) means that the widget will be skipped when tabbing through the form.
Default value: 0