Indicate when typing into a text control has no effect

[SaschaCowley]

Is your feature request related to a problem? Please describe

Users typically expect that typing into a text control (which is not disabled or read only) will insert text. However, this is not always the case:

• The application may have filtered the input, e.g. to mandate a particular input format.
• The maximum length of the input may have been reached.

Currently, NVDA provides no indication when these cases occur, so the user has no way of knowing that their typing has not achieved the expected result. To a sighted user, this is much more obvious, as they can see the text being inserted as they type.

Describe the solution you'd like

If the text at the caret has not changed after typing into a text box, indicate this to the user. This could look like:

• Playing a sound, such as Windows' default error sound, or a custom earcon
• Displaying some sort of icon or message in braille
• Speaking an error message (this is likely to be quite disruptive)

Describe alternatives you've considered

Do nothing

Additional context

This is likely to require some thought to get right. We need to make the indication of error easily noticeable but also minimally disruptive. Applications may also already communicate such issues to screen readers, though in my experience very few do.

Alternative to part of the proposals in #19182

Related to #7910 (this would fix that issue)

[CyrilleB79]

Speech feedback is not a problem.
It may be disruptive, but in any case, if you type somewhere and if the characters are not actually entered in the field, there is no point in continuing this action.