Mouseless selection not working properly in datebox and calendar

Multiple days are visually marked as selected when using arrow keys and mouse pointer (no clicking) at the same time. Screenshot attached. Both datebox and calendar components are affected.

How to reproduce:

• open a calendar, for example on the demo page.
• move your mouse pointer to "4", then click and don't move your mouse, leave it at "4".
• using the arrow keys change selection to "6".
• move your mouse pointer to "6" and don't click, just leave it there.
• using the arrow keys change selection to "8".
• move your mouse pointer to "8" and leave it there.
• using the arrow keys change selection.
  ... and so on, multiple days are selected.