When inserting text or deleting text is for updating composition string,
`InsertTextWithTransaction()` does not suggest caret position because in the
most cases, `CompositionTransaction` updating `Selection` properly. However,
if it's deleting existing composition (i.e., replacing the composition with
empty string), `HTMLEditor::HandleInsertText()` doesn't use
`CompositionTransaction` to update composition. Instead, it uses the suggested
caret position from `InsertTextWithTransaction()` even though it's always unset.
Therefore, if the composition string follows some collapsible white-spaces and
they are replaced, `Selection` is automatically moved to the start position of
the replaced range and `HTMLEditor::HandleInsertText()` does not maintain the
caret position.
Therefore, this patch make it collapse `Selection` to end of the inserted text
where the composition string was.
Differential Revision: https://phabricator.services.mozilla.com/D246087
fb1d603cfd