Korean keystoke is lost when a completion candidate appears in ComboBox.

* .NET Core Version: 3.1.403
* Windows version: 10.0.19041
* Does the bug reproduce also in WPF for .NET Framework 4.8?: Yes

**Problem description:**
When a ComboBox is editable and has some items, if I type something in Korean and a completion candidate appears, the keystroke gets lost. For example, if there is an item "고양이" and I have typed "고양", the moment I type "ㅇ", "이" appears as a candidate, but the stroke "ㅇ" is lost, so if I continue to type "l", it becomes "고양ㅣ".

**Actual behavior:**
Keystroke gets lost.

**Expected behavior:**
Keystroke should not be lost.

**Minimal repro:**
Run the following code, type "고양이".

```

고양이

```