prevent horizontal scrolling

Given an editable element with text content "foobarbaz" and selection range "bar", pressing the left arrow key five times in succession will produce the following results:

1.  `foo|barbaz` (selection range is collapsed)

2.  `fo|obarbaz`

3.  `f|oobarbaz`

4.  `|foobarbaz`

5.  `|foobarbaz`; an ancestor's horizontal scroll offset may be updated

This commit attempts to prevent horizontal scrolling without affecting normal text field manoeuvring.

Does this look okay to you, @techmec?