Operating system: Windows-10-10.0.17134 64 Bits
Graphics card: GeForce GTX 560 Ti/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 388.13
Broken: version: 2.80 (sub 74), branch: master, commit date: 2019-07-18 14:52, hash: rB38d4483c6a51
Short description of error
About Grid Fill option with circles (CTRL+F), in blender 2.79 you can choose the Span number, usually the option starts with a Span of 4, but you could choose a Span of 1 to make a simple "cap" for a cilinder for example, but in Blender 2.80RC2 you can't change the Span option, and the simple "cap" has to be made manually.
Exact steps for others to reproduce the error
Create a circle of 12 or 18 vertices, extrude to make a cilinder, then select the top row of vertices and create a cap with CTRL+F > Grid Fill. In the Grid Fill options, try to move the Span number to 1 or any other number, you can't.