ContentEventHandler::ExpandToClusterBoundary() doesn't check the return value of nsTextFrame::PeekOffsetCharacter(). Therefore, it may set its result to reversed offset. (e.g., when aForward is true and offset is 6, the result may be 5. When aForward is false and offset is 5, the result may be 6.)
For avoiding that, ContentEventHandler::ExpandToClusterBoundary() should check the result and only when it returns nsIFrame::FOUND, it should compute the proper offset.
On the other hand, it's too bad for ContentEventHandler that nsTextFrame::PeekOffsetCharacter() to return nsIFrame::CONTINUE_UNSELECTABLE when the user-select style is "all" because IME doesn't expect such cases.
Therefore, this patch adds additional argument to nsIFrame::PeekOffsetCharacter(), aOptions which is a struct containing bool members. The reason why it's not a bit mask enum is, such struct doesn't cause simple mistake at checking the value and the code is shorter. When mIgnoreUserStyleAll of it is true, this patch makes nsTextFrame not return nsIFrame::CONTINUE_UNSELECTABLE.
MozReview-Commit-ID: ACNNBTP92YZ
nsIFrame::mClass is of type enum class nsQueryFrame::ClassID which is
a strict subset of the nsQueryFrame::FrameIID values. For a concrete
frame class, its FrameIID is the same numeric value as its ClassID.
MozReview-Commit-ID: 1N0AkCGo1ol
