Input method doesn't handle focus out/ focus in correctly

On my Macintosh, input method doesn't do the right thing if I start typing in one window using an input method, activate another window, then go back to the original window. I have attached three screen shots showing the problem. They show two windows containing QPlainTextEdit widgets. Here is the description:
1. Select an input method. I used Japanese Hiragana.
2. Activate one of the windows and type "mit". The result is "みt". This is shown in the screen capture called "Step1_mit.png".
3. First bug: Click in the other window, then back in the first. The characters from the first window are now inserted in the second window. Shown in screen capture "Step2_focusInandOut.png"
4. Second bug: type "s". Now the window contains just "s" in the input method string. Shown in screen capture "Step3_s.png".

It appears that the first window loses its Input Method context at step 3. I would expect that the "s" typed in step 3 would be added to the existing preedit string. At the least, on focus out, the current preedit string should be committed. Instead the sequence I outline here results in a confused state.