When I open a new window after a DnD operation, IME does not work for that window.
I assume this issue is related JDK-8252470.
Please try following operations on Windows which has IME, like Japanese Windows:
1. Compile and run WinDnDTest
2. Press "Create New Window" button twice by mouse
2 windows are opened, Windows IME should work on these windows
3. Select some strings by mouse on one of window
4. Drag selected strings and drop it into another window
5. Press "Create New Window" button
Window is opened, but Windows IME does work on this window