Run the attached program.
  - Selecte 'READ TEXT' menu item, text is displayed.
  - Click on text in JTextArea and press 'End' key.
  - Input some characters.
Then, character input position is different from character display position.

End of text line contains ' ' fullwidth character.
Because, when you append text after end of line, input character is not displayed
at the current cursor position.

JTextPane is the same.