xforms-previous event doesn't always happen when it should

When navigating between between controls with the 'tab' and 'shif-tab' keys, the xforms-previous-event is not always dispatched when it should be. The attached form demonstrates the bug. Do the following in the form:

- Open form. Focus is given to the first input control.
- Tab through the controls to the last one. Each tab inserts a log message into the inst-log instance (shown at the bottom of the screen) on the xforms-next event
- When focused in the last control, shift-tab back to the fourth control. An xforms-previous event happens as expected and a log message is inputted into the inst-log instance.
- Shif-tab again and the expected xforms-previous event does not happen. xforms-next happens instead.
- Another shift-tab generates an xforms-next event too.