ctrl-b weirdness when there is leading tabs

Simple test case:
- run make
- run libnoise_example
- press tab _twice_
- type foo
- press space one or more times
- type bar
- press ctrl-b

You'll see that rather than moving the cursor back over the r in bar, it moves it to the space before the f in foo. On top of that, if you start pressing ctrl-f, you will eventually be unable to move past the f. This is because the cursor _seems_ to be at the end of line already. Any thing typed after hitting that invisible wall, will be echoed out as occuring after the bar when you press enter.

This seems to be a regular thing when lines have tabs in them during a ctrl-b/left arrow event.