Session files should always keep working. At least upwards. Not downwards.
However. IIRC there have been session files that got broken for unknown reasons. Maybe there is an undiscovered bug, that sometimes writes a broken session, I do not know. If a broken session files gets loaded by one version, there is no guarantee that it will be by the next version.
As for your particular session files, regarding Anchordocking, I dont know if they are ok or broken. I don't use anchordocking, and I don't maintain it either.
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I havent looked at the code, but
"windowindex" of -1
might be a bug.
(EditorIndex -1 seems to exist though)
At least none of my sessions has it (and I use 2 source editors all the time).
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As I said, not sure if this is a bug, If windowindex -1 is a bug, then fixing it should include to read the broken session...