Making it async causes odd behavior of `TSFTextStore`. I guess that it's caused by `TSFTextStore` may create its instance at `SetInputContext` too, but cleaned up at `NOTIFY_IME_OF_BLUR`. [This hack](https://searchfox.org/mozilla-central/rev/43cb6eca1c3069d46d589d52ab4949257e630d19/dom/events/IMEStateManager.cpp#242-244) makes me suspect this. I keep investigating...
Bug 1760045 Comment 18 Edit History
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Making it async causes odd behavior of `TSFTextStore`. I guess that it's caused by `TSFTextStore` [may create its instance at `SetInputContext`](https://searchfox.org/mozilla-central/rev/43cb6eca1c3069d46d589d52ab4949257e630d19/widget/windows/TSFTextStore.cpp#6664-6671,6694-6700) too, but cleaned up at `NOTIFY_IME_OF_BLUR`. [This hack](https://searchfox.org/mozilla-central/rev/43cb6eca1c3069d46d589d52ab4949257e630d19/dom/events/IMEStateManager.cpp#242-244) makes me suspect this. I keep investigating...