Pull Request comment field doesn't handle Japanese IME input properly

Issue Summary

Activity comment field in Pull Request screen doesn't handle Japanese IME input properly.

Steps to Reproduce

Case 1

1. Open Activity panel in the Pull Request screen
2. Set IME to Japanese (I tested with Mac Hiragana input)
3. Type A key four time, it will create "ああああ" which is equivalent to "aaaa"
4. Type Enter (to submit the text as "ああああ")

Case 2

1. Open Activity panel in the Pull Request screen
2. Fill 4 lines in the text field and make the 4th line long to reach to the right side of the input box

   a
   b
   c
   dddddddddddddddddddddddddddddddddddddddddddddddddddddd
   

3. Set IME to Japanese (I tested with Mac Hiragana input)
4. Set the cursor in the end of 4th line and repeat typing A key which will generate "あああああ......."

Expected Results

Case 1
In the step 3 above, only the characters "ああああ" is shown and placeholder text disappear.

Case 2
The field accepts repeated A key input "あああああ......." and just keep adding the character based on the input.

Actual Results

Case 1
In the step 3 above, the character is overlapped with placeholder text.

Case 2
The repeated A key input make the IME input shown and disappear.
Screen Recording 2020-08-06 at 11.52.50.mov

Workaround

No workaround found. Source code need to be modified.