[resize-observer] Why does ResizeObserver not fire on non-replaced inline elements?

The resize-observer spec says

Observations do not fire for non-replaced inline Elements.

I am guessing this is because such elements are split into pieces (rows).

I see that the latest ResizeObserver spec allows it to return an array of sizes when content is split across columns. Based on this, it seems feasible to also have it give us an array of the sizes of the pieces of a non-replaced inline element's rows.

Or it could just give us the size of the bounding box, but this would make the use cases more limited, though the limited use cases could still be useful.

Is it simply that nothing has been decided on how to handle multiple pieces of a non-replaced inline element?

Start with the Resize Observer specification linked in the issue and read the rule excluding non-replaced inline elements alongside its handling of fragmented content. The issue asks whether reporting sizes for inline-element fragments or a bounding box is appropriate, but identifies no implementation files or tests; done would require a settled behavior and corresponding specification changes.