ui: cursor column for dir=up/down is broken when at the empty string at EOF after a \n

Moving up/down with the directional arrows attempts to keep the cursor in the same column. If the cursor is at column=0, you cannot move down to the empty string at the end of the file, just after a newline.

To reproduce:

create a textbox with the text `Hello, World\n`. Move the cursor to just before the "H" and press down. The cursor moves to just after the "d". We expect it to be just after the "\n".