What is the best way to implement character counter located in separate FormField?

I have tried StaticItem with formatValue observing target TextItem.changed and calling redraw each time, but I know that it is not a way to do it as it causes all fields to redraw including the one that is edited. On mobile it also causes very weird behaviour where contents of TextItem is being doubled after long pressing any button.
Here it is shown with backspace: https://demo.smartb2b.eu/long-press-bug.mp4 - it is recorded on v11.1p_2019-11-25/LGPL

Best regards,