When committing composition string,
`WhiteSpaceVisibilityKeeper::InsertOrUpdateCompositionString()` may
replace commit string with normalized white-spaces. Therefore, if
the last character is a white-space, it may be replaced with the other
white-space (ASCII white-space vs. NBSP). Then, `Selection` collapsed
after the last character will be moved to start of the replaced
white-spaces. So, when committing composition,
`HTMLEditor::HandleInsertText()` may need to change `Selection` by
itself.
Original Revision: https://phabricator.services.mozilla.com/D252278
Differential Revision: https://phabricator.services.mozilla.com/D253280
f3c6a960d9