Reproduces the frame split of the Salesforce Classic email composer for
issue #10379
Phase 2: the CKEditor 4 editable body is a nested about:blank frame inside
an /email/htmlbody iframe, while the Send button is here in the top
frame. No single frame owns both the editor and the button, so a same-frame guard can't
block the send — the extension mirrors the body's verdict up and blocks at this button.
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