Color picker does not change the color of a Vector Parameter in a Material Instance until the mouse button is released

This is a regression from 4.23.1 (CL-9631420). The color picker does not update in realtime anymore when holding down the left-mouse button. Only on mouse-release does it update.

This was reported and tested in 4.24.3 (CL-11590370). This was reproduced in 4.25 Preview 2 (CL-12055273) and Main 4.26 (CL-12063615).

1. Open the UE4 Editor and create a Blank project
2. Create a Material and open it
3. In the Material editor, create a Vector Parameter and hook the RGBA output to BaseColor, then save the Material
4. Create a Material instance of the Material
5. In Material instance editor, check the box of Vector Parameter and change its color using color picker by holding down the left-mouse button

Results: The color is not updated in the preview and viewport until the mouse button is released

Expected: The color is updated during an edit