Thanks for the clean reproduction project.
This is by design as any way on handling rounding will cause problem. The current algorithm guarantee that the total of the element will be filled if the percentages add up to 100% and has no corner case handling as they usually introduce more quirk than benefit.
Here are two avenue to mitigate this :
- Manually drive the scaling to be whole numbers of 1 at constant pixel size. This usually help a lot with having a predictable result
- You could try setting the width (and left) explicitly instead of setting top and left. Maybe specify the widths in scaled pixels instead of % would also help a lot.
Finally in unity 6 we exposed some utilities to help the users with the rounding, (see AlignmentUtils.FloorPanelPixelSize). You could possibly make some adjustments to the size of the element in the geometry changed event manually, but that would be a real last resort.