FIX: Remove mouseup listening when drag begins (closes #460)
FIX: Remove mouseup listening when drag begins (closes #460)

charnpreetsingh185/jupyterlab,jupyter/jupyterlab,jupyter/jupyterlab,jupyter/jupyterlab,jupyter/jupyterlab,charnpreetsingh185/jupyterlab,TypeFox/jupyterlab,TypeFox/jupyterlab,TypeFox/jupyterlab,TypeFox/jupyterlab,jupyter/jupyterlab,TypeFox/jupyterlab,charnpreetsingh185/jupyterlab,charnpreetsingh185/jupyterlab,eskirk/jupyterlab,charnpreetsingh185/jupyterlab,eskirk/jupyterlab,eskirk/jupyterlab,eskirk/jupyterlab,eskirk/jupyterlab