Screencast 2 (Attachment 9259807 [details]) shows the current behavior on TB 91 old AB for Ctrl+drag of unselected contact with another selected contact (drag only the selected contact). I thought I had the rationale of this edge case behaviour covered in my comment 5, but it turns out Windows 10 Explorer behaviour (which may have some de facto normative value) is different. Ctrl-drag will select the second item too before dragging both items with Copy flavor. So Ctrl actually has two functions here, and Windows Explorer went for applying them both at the same time.
Bug 1732553 Comment 12 Edit History
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Screencast 2 (Attachment 9259807 [details]) shows the current behavior on TB 91 old AB for Ctrl+drag of unselected contact with another selected contact (drag only the selected contact). I thought I had the rationale of this edge case behaviour covered in my comment 5, but it turns out Windows 10 Explorer behaviour (which may have some de facto normative value) is different. Ctrl-drag will select the second item too before dragging both items with Copy flavor. So Ctrl actually has two functions here, and Windows Explorer went for applying them both at the same time. Which may or may not be intended.