All Popou controls have been desabled by default. You can enable it from Settings. Please follow the below steps to enable/disable all popup controls (calenders, Task, Baseline controls) in Data sheets.
Pleasse note, Undo command will not work in the data sheets when this option is ENABLED.
Goto Settings
Goto Advanced Options
Select the Popup Controls in Datasheets option as ENABLE
And Apply the settings
Please see the below screen-shot for pictorial representation of the above settings: