In kmail (again) I found another bug:
(I created a test case for it)

Load test case
Select text => click on "show selection" button
=> you see selected text
=> ok

Now click on 'switch html' we will change html with setHtml

=> you will not see selection

=> click on "show selection" => you see old selection.

For me it's a big bug for kmail when we reply to an email which selected text
=> we will reply with old selection.