If a selection range set via setBaseAndExtent starts or ends with collapsed whitespace, it is excluded from
the range, which doesn't happen in Firefox.
Created attachment 5446 [details]
Here's why this happens:
Positions passed to Selection::setBaseAndExtent are used to construct VisiblePositions. VisiblePositions
take the input position and find its canonical position: the position that is visually equivalent but "deep"
and at a rendered offset (this canonical position is referred to internally as a "deep equivalent" and is
returned by VisiblePosition::deepEquivalent()) For example:
A VisiblePosition created with the Position (div, 0), would have a "deep equivalent" of (textnode, 0).
A VisiblePosition created with the Positions (textnode, 6), (textnode, 7), and (textnode, 8) would all
have deep equivalents of (textnode, 8).
The behavior you see is caused because SelectionController takes the Positions passed to
setBaseAndExxtent, creates VisiblePositions, then sets the base and the extent to the deep equivalents
of those VisiblePositions.
SelectionController does even more fiddling with its positions. When it creates start/end (from base/
extent), it upstream()s the end and downstream()s the start. Comments in SelectionController claim
that this is necessary to "constrain" the selection to the fewest possible nodes, in order to make it
canonical for purposes of comparison with other selections. I'm not sure why this is necessary, since
VisiblePosition's are already canonical.
Both of these things need to be addressed to fix this kind of bug (when ranges change when they are
used to make selections).
See also: bug 15256.