Modify slider style on line count change instead on adjustment signal

Tries to workaround https://gitlab.gnome.org/GNOME/gtk/-/issues/7382

Introduces a minor regression: When the input shows > 1 line and gets resized so that the whole text finally fits into a single line, the view's height is 2 lines tall, until the text (buffer) is edited.

Otherwise it worked fine in practice for me and in my testing, I could not trigger the bug anymore.

Alternative to !1126 (closed)