Alt-kana/rome on JApplet/IE7 makes IME/TextField unusable

Windows Vista, IE7 + 1.5.0_17

After starting a JApplet on IE7, moving focus to a JTextField, then enable IME. When IME
is active, keyboard input mode can be changed between rome-character mode and kana-mode
by Alt-Kana/rome key on Japanese 106 keyboard (or screen keyboard utility). On Vista,
this operation shows an Windows IME dialog which confirms whether user is really
switching to the different keyboard input mode. Select yes to change it.
 
If this was done several times while a JTextField has focus, IME/TextField becomes
unusable. Windows language-bar, which shows IME status disappears too. It is shown
again while moving focus to other window on the screen.

Once this happens, if I type some keys after this problem, I see some IE7 dialog (as if the
key sequences are handled as a short-cut key) or orphaned IME dialog at the top-left
corner of the screen.

The same operation does not show the symptom on XP.

So far, the problem does not happen with a stand-alone Swing program.
I don't see the issue with Firefox 3.5.3, Safari 4 and Chrome 4, either.

The same symptom was seen on Vista+IE7+6u18 jre.
.