"Send email" produces double "Processing" button

Steps to reproduce:

1. In "Contacts" menu, click "New Email"
2. Enter a recipient and subject
3. Click "Send"
4. Observe: an extra button appears with the text "Processing" (attached)

The length of time for which the text displays depends on how long submission takes. To see it reliably, add a hack to slow the request processing (e.g. in CRM_Core_Invoke::invoke, add "sleep(15);").

The problem is observed on the current master code. I reproduced on sandbox, but some notes:

• When I first checked sandbox, it claimed version 4.4.beta1 (aka the previous version) and did not have the bug.
• When I updated the sandbox, it claimed version 4.4.beta3 (aka the upcoming version) and did have the bug.