System Information
Win7 64bit

Blender Version
Broken: 2.79

Short description of error
If place Scene strip with Alpha Over or few others blending (except Cross), keyframe value of size can be edited only in Graph Editor. But with Cross blending everything is fine.

Exact steps for others to reproduce the error
Open file, set Scene strip blend to Alpha Over and change keyframe value size in Gauss blur strip-> fail
set Scene blending to Cross and change value -> OK