If a user is changing between tabs and spaces, and they
have already typed 2 spaces, then want to tab the rest
of the indentation, the tab does not complete the
indentation as expected.
For example, if tabs are set at 4 spaces, and a user
has entered 2 spaces, most users expect that hitting
tab will bring them up to 4 spaces. But instead, the
tab will be placed before the spaces, and the extra
spaces will cause problems with indentation. This is
best seen with the 'view white space' option on.