Summary: There is a long standing issue about the event order of
`|compositionend|` and `|input|` at committing composition. Gecko
dispatches them as:
1. `|beforeinput|` whose `|isComposing|` is `|true|`
2. `|compositionend|`
3. `|input|` whose `|isComposing|` is `|false|`
However, Chrome dispatches them as:
1. `|beforeinput|` whose `|isComposing|` is `|true|`
2. `|input|` whose `|isComposing|` is `|true|`
3. `|compositionend|`
The reason why we cannot align this behavior to Chrome is, there is no
`input` event whose `isComposing` is `false`. Therefore, web apps cannot
handle `input` events only with `input` events to avoid to do something
during IME composition, i.e., web apps needs to listen
`compositionstart` and `compositionend` for doing that. So, once we
align the behavior to Chrome as-is, we may need to rewrite our chrome JS
code with more event listeners. Therefore, I filed a spec issue
<https://github.com/w3c/uievents/issues/202> several years ago, but it's
still not agreed between browser vendors.
However, now, we have a web-compat report
<https://bugzilla.mozilla.org/show_bug.cgi?id=1910865> which is caused
by this event order difference. The web site explicitly sets
`|HTMLInputElement.value|` at every `|input|` event except during IME
composition. For supporting IME, they set the value from
`|compositionend|` too with new value which is stored at the last
`|input|` event. Therefore, they set previous composition on Firefox if
the user selects a commit string from the candidate list without
updating the composition string.
For avoiding the web-compat issue on the web site, I think Gecko can
dispatch redundant `|input|` event before `|compositionend|`, so,
1. `|beforeinput|` whose `|isComposing|` is `|true|`
2. `|input|` whose `|isComposing|` is `|true|`
3. `|compositionend|`
4. `|input|` whose `|isComposing|` is `|false|`
So, the new behavior causes alone `|input|` event which is not following
a corresponding `|beforeinput|` event, but I guess this does not cause
new web-compat issue.
Bug: <https://bugzilla.mozilla.org/show_bug.cgi?id=1941973>
Specification: UI Events
<https://w3c.github.io/uievents/#events-composition-input-events>
Platform coverage: All
Preference: `|dom.input_events.dispatch_before_compositionend|`
DevTools bug: N/A
web-platform-tests: N/A (cannot synthesize composition with TestDriver)
--
Masayuki Nakano<[email protected]>
Working on DOM, Events, editor and IME handling for Gecko
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