When typing into rsyntax area, JLabel's within the same vertical panel move.
See https://github.com/daveray/seesaw/issues/133 for a java example and video of said bug.
This is not a bug, but rather a case of funky behavior with BoxLayout. The same behavior can be seen in your sample program using just a JTextArea, no RSyntaxTextAreas involved.
I suggest putting the JLabel in a JPanel with BorderLayout, at BorderLayout.LINE_START, to force left-aligning of the label.