System Information
Operating system: Linux-5.4.0-28-generic-x86_64-with-debian-bullseye-sid 64 Bits
Graphics card: GeForce RTX 2070 SUPER/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 435.21

Blender Version
Broken: version: 2.83 (sub 15), branch: master, commit date: 2020-05-02 11:33, hash: rB1623fdb3bc55
Broken: 2.82

Short description of error
The Opacity slider under Adjust > Compositing on the Strip tab in the VSE Sequencer Sidebar doesn't work as expected with certain blend types.

It works as expected with Add, Cross, Multiply, Over Drop, Gamma Cross, Subtract, Alpha Under, and Alpha Over.

However, it functions as a switch (0 = off, >0.000000001-1 = on) for Overlay, Soft Light, Hard Light, Vivid Light, Color Dodge, Screen, Lighten, Linear Burn, Color Burn, Darken, Value, Color, Saturation, Hue, Exclusion, Difference, Pin Light, and Linear Light.

(This may very well be the expected behavior for some of these, I'm not an expert on blend types. Also, I am unsure what Replace is supposed to do, so I didn't include that above.)

Also, the Normal blend type (opaque) seems to be missing. Alpha Over seems to be the default for any new strips that are added.

Exact steps for others to reproduce the error
Start a new file by selecting Video Editing. Add two images or movies to the Sequencer window, and place these strips on top of each other. Under Adjust > Compositing on the Strip tab in the sidebar, change the blend type of the upper strip and play with the Opacity slider, going between 0 and non-zero values.