Update Y-sort position of the first item in the sorted subtree

[kleonc]

Fixes #79451.

Also updated the docs as Y-sorting affects not only the child nodes, the CanvasItem with y_sort_enabled = true is being Y-sorted together with its children (always was, in a potentially buggy manner but still).

[akien-mga]

Tentatively approving, as the changes seem simple and I trust @kleonc's assessment. But I'm not competent on this rendering code so a review from Clay or lawnjelly would still be very welcome (but if they're busy, I'm happy to yolo merge this in a few days and we'll see in production if any edge case was forgotten).

[clayjohn]

I'm not really sure about this code as I'm not familiar enough with the surroundings to know why the parent node was excluded from the y-sort in the first place, but I trust kleonc, so let's go ahead with a yolomerge

[kleonc]

I'm not really sure about this code as I'm not familiar enough with the surroundings to know why the parent node was excluded from the y-sort in the first place

AFAICT the docs being incorrect about that is a result of them being simply a reworded version of the old YSort node's description when it was changed in #42282. But in that PR the canvas items with y-sort enabled were made to be included within the y-sorting themselves so it would work properly.
In 3.x it seems to me like the VisualServer part was written taking in mind specifically a separate YSort node which were assumed to not draw anything on its own, and hence it was skipped within y-sorting. Not sure whether it was intentional or was just overlooked but there's e.g. #49165 which seems to be caused by this.

Also by always was I guess I meant in 4.x 🤔:

Also updated the docs as Y-sorting affects not only the child nodes, the CanvasItem with y_sort_enabled = true is being Y-sorted together with its children (always was, in a potentially buggy manner but still).

[akien-mga]

Thanks!