1. Open Claws Mail.
2. Compose a new message.
3. Type "abc" in it.

Then (strange but sorry I couldn't find a simpler situation):
4. Open a site in Chromium.
5. Open Developer tools (<Ctrl>+<Shift>+I).
6. Right clink on some DOM element.
7. Click Copy -> Cope element in context menu.

Say, element's HTML is: <p>def</p>

8. Switch back to new message.
9. Select all text.
10. Paste (<Ctrl>+V).

Expected message's text: <p>def</p>
Actual message's text: <p>def</p>abc