Guard editor scroll position across Enter

Pressing Enter mid-document was reported to jump the document scroller
toward the bottom. The jump came from macOS Chromium binding End to
scrolling the document, which the reporting test pressed before Enter.
Enter itself, including a slash command's Enter, leaves the scroller still.

Add browser coverage for both halves: Enter mid-document keeps the caret
line in place, and typing past the bottom still scrolls the caret into view.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>