Hello Brian,
Thank you for writing.
The multiple selection should be kept no matter if you click a date cell or the header cell. I have logged it in our
feedback portal and I have added a vote for it on your behalf. You can track its progress, subscribe for status changes and add your comments on the following link -
feedback item.
I have also updated your Telerik points.
Currently, the possible solution that I can suggest is to use the MouseDown event to store the currently selected cells and then on MouseUp restore the missing selection:
I hope this information helps. Should you have further questions I would be glad to help.
Regards,
Dess
Progress Telerik