the defaults for new editors were being set for the highlighter codemirror option, but not for the mark-selection addon, which is required for the selection not to be highlighted. As a result, cells created after the nbextension loaded would highlight selected text (might have been behind #10?). In addition, the selected text in edit-mode (non-focussed) cells was still being highlighted, so this fixes the css rule for that.
the defaults for new editors were being set for the highlighter codemirror option, but not for the mark-selection addon, which is required for the selection not to be highlighted. As a result, cells created after the nbextension loaded would highlight selected text (might have been behind #10?). In addition, the selected text in edit-mode (non-focussed) cells was still being highlighted, so this fixes the css rule for that.