Currently, LayoutNG has a very simplistic approach to this: Constrain available inline size on an orthogonal flow root to any max-block-size of its container and the initial containing size, whichever is smaller.

The spec [1] has one more rule; to constrain to the ancestor scrollports. NG still doesn't do this. The benefit weighed against the cost (memory) is considered to be low.

Additionally, other engines also respect a fixed block size set on some ancestor (Blink legacy, WebKit, Edge) [2], and subtracts block margin+border+padding specified on the ancestors (Blink legacy and Edge) [3]. This is non-standard behavior, though.

[1] https://www.w3.org/TR/css-writing-modes-3/#orthogonal-auto
[2] demo-orthogonal-fixed-ancestor-height.html
[3] demo-orthogonal-body-margin.html
 

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