This happens only with HTMLEditor. TextArea and TextField work fine.

Steps to reproduce:
1. Start a HTMLEditor, invoke Chinese or Japanese input method.
2. Compose a word, then commit.
3. Compose another word, then highlight the partial composed text.
4. Do not release mouse, highlight the the committed text in step 2 as well.
5. Continue to compose other words.

There are 3 problems happen during the steps.
1. In step 3, the highlighted composed text is not visible. (the background is white)
2. In step 4, after the committed text is highlighted, the partial composed text got committed. (no underline anymore)
3. In step 5, composed text is not visible.

Please refer to attached video htmleditor-im.avi. Test platform is Windows 7, haven't try on Mac and Linux yet.
It could be an old problem, also reproducible with FX 8.

Highlighting composed text is not a common action when using input method, so the impact could be limited.