The ckeditor looses its focus when switching between tabs and using the arrow keys.
Steps to reproduce:
Open the ZK Sandbox (http://www.zkoss.org/zksandbox)
Open the "WYSIWYG editor" where the ckeditor is shown
Click on the second Tab "View Source"
Click on the first Tab "Demo"
Click into the editor editing area to position the cursor.
Use arrow key left/right --> focus lost.

Now, if you would press Back, the browsers goes back.

I think that it has something to do when the ckeditor is placed in an tabbox and you sitch between different tabs.
In the zk-demo (not the sandbox) where no tabs are used, the editor does not have this problem.