1) Ctrl+Z keyboard shortcut for Undo.
2) "Save Left", "Save Right", "Save Both" buttons. Or alternatively "Save Current Pane" and "Save Both" buttons like in Araxis Merge. When Shift is pressed they can turn into Save As buttons.
3) "Save" and "Save As" to context menu.
4) Redo button.
5) Ctrl+Shift+Z keyboard shortcut for Redo.
When user selects line only in one panel and presses "Resync" the error message box is displayed (with red cross icon). I think it's too frightening (I almost got heart attack

"Resync comparison from selected lines" sounds more correct than "Resync comparison from here" because you can right-click not on selected lines but in some other place and it's a little confusing.