Came across a similar issue that I was able to solve. In my case, even though I was adding a ton of content to my HBox (that was inside a scroll container and without clipping enabled), the size reported in Remote (debugger) was the same size as the gridcontainer which is unexpected. The HBox should have been way bigger. I ended up having to measure the width of all children in my HBox and then set the HBox min_size to the combined total. For me, I did this on resize but it could be done on add or wherever.
To see if this is your issue, look at remote and compare your gridcontainer dimensions with it's top control node dimensions. If they are the same, this is likely the problem.