Make waving cover files around on boards more reliable master
Summary: Currently, in Safari, if you drag an image onto a board to make it a cover file and then wave it around wildly a lot over differnent cards, it sometimes glitches out a bit and won't drop on them properly. This appears to be because sequencing and delivery of dragenter/dragleave events isn't always totally ideal. Instead, just cancel any existing drag when we get a new drag that targets a new drop target. Test Plan: - Opened a board with a bunch of cards. - Dragged a file from my desktop onto the board. - Waved it around wildly, hovering over many different cards. - Before patch: sometimes cards under the cursor stopped highlighting properly. - After patch: behavior seems correct and consistent. Reviewers: chad Reviewed By: chad Differential Revision: https://secure.phabricator.com/D15208