Subject: bug#39098: 26.3; bug of mouse-face including linebreak

From: Eli Zaretskii <[email-masked]>
Date: Mon, 13 Jan 2020 10:01:49 +0200

On January 13, 2020 8:53:07 AM GMT+02:00, [email-masked] wrote:
> 
> Why is the out-of-range character highlighted only if it is at the
> beginning of line?

Because you've covered the newline with mouse-face.  Newlines leave no glyphs on display, so the next character gets highlighted instead.  If you don't want this, exclude the newline from the mouse-face propertized part, it is unnecessary anyway, because a newline can never be "mouse-sensitive".