If I trick textarea/textinput to accept control characters in its input (most easily, by using control characters in the input of SetValue, but also possible via bug #466, textarea/textinput get confused:

• the width computation for the control characters is wrong
• navigating the cursor from a line containing control characters to the previous/next line can cause the bubble to panic (because the width computation is wrong)

This is most noticeable when inserting a Tab character (ASCII 9) but some other control characters also display weird behavior.

FWIW, the patch in #214 fixes this by using the (new) function tea.SanitizeRunes on the input string. I'm not sure that's the best approach, but it works.