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`ContentEventHandler` may not replace open/close tags of inline elements and close tags of block elements with a line break etc. Therefore, one offset may mean multiple points in the DOM tree. From the DOM tree point of view, every DOM point may have different meaning and styles. Therefore, if IME sets `Selection` to current range, we should not update `Selection` with the log of `ContentEventHandler`. Otherwise, new content will appear at different position in the DOM tree and it may cause different style text, etc. `IMEContentObserver` has cache of `Selection` while an editor has focus. Therefore, for avoiding the query cost of `Selection` for each `eSetSelection` event, `ContentEventHandler` should be used in `IMEContentObserver` instead if an instance is available. Therefore, this patch makes `TextComposition` delegate the handling to `IMEContentObserver` and check `Selection` range only when `IMEContentObserver` is available. Note that `eSetSelection` is probably used only when editable element has focus. Therefore, it must be fine to skip selection range check in the other cases. Differential Revision: https://phabricator.services.mozilla.com/D171179