Bug: bad cursor position in text input

[dcz-self]

1. Set up a text field with a placeholder
2. Set up on_enter which clears the text
3. Run app
4. Type some text
5. Press enter

Expected: cursor is at the start

Actual: Cursor is in the middle of the placeholder.

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I'd fix it myself but I'm tired right now. Filing so that it doesn't get forgotten.

This causes bad interactions wth set_surrounding_text btw, but at least nothing crashes, even when the cursor falls in the middle of a code point.