Pre-Edit string shows buggy behaviour when used with Japanese (Hiragana)

On Focus change pre edit string disappears. Attached example reproduces the behavior.

Steps to reproduce

• In the 'Region and Language' select 'Change keyboards..' Add Japanese Microsoft IME Keyboard. (or run on Japanese edition of Windows)
• Run attached sample.
• From the Language Bar change the input language to Japanese. Also change the Input mode to Hiragana (it has the pre-edit state). Now the taskbar should have the icons shown in the taskbar.png
• Click a line edit and press 'a' key five times. (inputed "あああああ”in Japanese)
• Click another line edit and press 'a' once, six "あ" will appear unexpectedly.
• Click first line edit again, "あああああ" inputed will disappear