Block [Column] selection not working as expected

When I select a block of text and type a new string, the selected text is deleted from all the rows that the selection covers, but the new text is only inserted in the first line.

For example if I select the block

   bbb      in      aaabbbccc
   bbb              aaabbbccc
   bbb              aaabbbccc

 then type xxx

I expect it to become
    aaaxxxccc
    aaaxxxccc
    aaaxxxccc

but instead it becomes
  aaaxxxccc
  aaaccc
  aaaccc

Is there some property I need to change in order for it to work the way other applications work?

(Note that I verified this in your Syntax editor demo app just in case it was caused by something my own app does.)