Great, thanks!

A first step could be to add the capability to have spaces inserted in place of a tab when the tab key is pressed.   Existing tabs, or pasting of tabs would not be affected.  This is a feature in the majority of editors support.   Changing all tabs in the file to spaces could come later.

The tab size should control the number of spaces inserted.   Since there is already a setting for tab size in the "File Compare Windows: View-Settings", it should be very easy to add a radio button to select "insert spaces for tabs".   Note, that I would think these tab settings should be at the top level menu under "Edit->Preferences".