I have managed to reproduce the bug, but I can't find a way to reliably do so. It seems to be somehow related to locking issues.
When working correctly, dragging a URL causes the cursor to become a small corner and, later, a small icon appears representing the dragged content. When this bug hits, the small icon never appears and the cursor stays in the small corner shape forever. The OP's report about high cpu use seems to be unrelated and caused by the page he was viewing, which has some sort of neverending JS effect.
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