I've never run into that, but I made my email window larger than default a long time ago (resized the window just as you would any other) so I wouldn't have noticed the claim button being hard to get at.
It doesn't like to stay resized for me, attached or unattached. PITA to have to continually resize it, but what can I do?
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Biggest Troll on the forums? I'll give you a hint:
I was having a lot of trouble claiming items I emailed to my global. The following workaround has made claiming fast and a one click every time for me.
Use the little blue button to detach the e-mail window from the chat area.
Drag the email window away so the claim button does not overlap the chat area.
Now click the claim.
Works like a champ for me. Hope this helps others too.
Jak