Case 1:
Add a new symbol, eg 'test'
Click in the replacement edit field (notice that there is no cursor)
Try typing (notice that the field receives no text)

Clicking in the replacement edit field should result in a cursor, and it should receive text.

Case 2:
Select 'test' from the symbol list
Press alt+r to focus the replacement edit field (notice that there is no cursor)
Try typing (text IS received)
Click in the symbol list (notice that the 'test' field is not updated)

Using the mouse to remove focus from the replacement edit field should result in the symbol item being updated.

These issues make the dialog difficult to use with a mouse.