bIsComposing assert with Japanese MS-IME input(candidate list related)

When using MS-IME as Japanese input method, scrolling out the text field out side the view causes bIsComposing to be set false - this state causes instability of the editor.

Crash happened with Windows7/Windows10, 4.14 and 4.15.

0. Make sure you are using MS-IME as Japanese input.
1. Create a default level if there is none
2. Tweak the layout to make the details panel height shorter
3. Select PlayerStart
4. Start inputting some Japanese in Player Start Tag and display the candidate list
5. While the candidate list is displayed, scroll the detail panel to make the Player Start Tag outside the view
6. (If you have attached VS to the editor process, you'll see check for bIsComposing is failing)
7. In 4.14, step 5 crashes the editor. In 4.15, try to open the help menu - the editor crashes with Assertion failed: bIsComposing [Link Removed]