Frontend's _dragging is not recognized by the backend and fails set_state()

[xiaochen-db]

Field _dragging was added to frontend's LeafletMapModel at #339, however it's not found by the Python backend during set_state(), and thus throws traitlets.traitlets.TraitError: Class Map does not have a trait named _dragging.

Note that this can only be reproduced when env var JUPYTER_WIDGETS_ECHO is set to true.

Looking at where _dragging is used, it seems to be only within the frontend. Can we change it to an attribute rather than the current model field, so that it won't be synced to the backend?

[xiaochen-db]

Also this jupyter-widgets/ipywidgets@eddfdc6 has been merged to ipywidgets so that during set_state() _dragging from the sync message can be ignored (because it's not a field to sync in class Map). But still it's worth not sending _dragging to Python at the first place

[jasongrout]

Can we change it to an attribute rather than the current model field, so that it won't be synced to the backend?

That was actually a suggestion on the original ticket that was never implemented. +1 to changing the model _dragging value to a view this._dragging attribute.

[jasongrout]

Note that this bug means that ipyleaflet is broken in ipywidgets 7.7.0 when the widget echo is turned on, and is also broken in master currently. We're fixing ipywidgets to be more lenient, but as @xiaochen-db says, i think it is good to fix it here too.