bug#47603: comint sets '(rear-nonsticky t) on entire prompt, disabling 'cursor-intangible

[JD Smith <jdtsmith@gmail.com>]

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Comint sets ‘(rear-nonsticky t) on the entire detected prompt text, which prevents `cursor-intangible from functioning correctly when applied to the prompt.  As Stefan put it:

I suspect the issue comes from the difference between `get-pos-property` and `get-char-property`: positions (like `point`) are not placed on a character but between two characters.  But text properties only apply to characters.  So the properties that are "on a position" are based on what properties would a character inherit if it where inserted at that position.

By default text properties are front-nonstick and rear-sticky, so basically a position gets its properties from the char right before it. But if you set (rear-nonsticky t), then you get no properties at all at that position.

This causes issues when "applying (rear-nonsticky t) a bit too generously, e.g. to all the chars in the prompt rather than only to the last one."

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