User Agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:47.0) Gecko/20100101 Firefox/47.0 Build ID: 20160413135321 Steps to reproduce: 1. Put the browser is under heavy load, such as when it's restoring a lot of pinned tabs 2. Click a tab to focus it 3. Move the cursor elsewhere Actual results: Often, this results the clicked tag unexpectedly popping out into its own window as if I had pressed the mouse button when I clicked, but then didn't release it until Firefox caught up its event handling. (ie. Firefox seems to be detecting drags based on the mouse's position when the button-release event was processed/received, rather than when it was generated) Expected results: Firefox should have registered no motion during the click process.
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