On a modern webbrowser (tested OS X 10.10, Chrome 38.0.2125.104), with master as of v2.8.0-beta-253-g8950e7f ...

• Set up a course with grades, such as the 'Glorious gradebook' course in qa.moodle.net
• Ensure AJAX Gradebook is enabled
• Open the Grader report
• Click on a grade cell to edit, edit value
• Hit left arrow, or tab keys (or down arrow),
• Expected: Saves edited value and moves to next editable value to the right (or down) from current "cell" being edited.
• Actual: arrow keys do nothing, tab key follows hyperlink tabbing order, going to first student name/hyperlink.

Perhaps tabs are not appropriate here. "Arrow keys" would be a workable navigation option.