Question Unity Frame Debugger doesn't clear opaque pass when Camera clear flags are Skybox

Here is something I could never understand:

Why should Camera clear flags affects how Frame Debugger display opaque pass?

- With clear flags set to Skybox, the opaque pass appear uncleared, ie. opaque pass is visible, then we render everything again.



- With clear flags set to Solid Color, the opaque pass starts with solid color, arguably my expectation.



- I understand why Skybox renders after opaque pass.

- My questions are why isn't the previously frame cleared? Is it that expensive to clear? Wouldn't clearing it make debugging easier?

This always feels like a bug to me, and I remembered a time when this behaviour isn't the default.

Can someone explain to me if this is a bug or not? As it has been the case for 2019 release cycle?

Thx!

 

Same problem in Unity2020