Popups don't scale linearly if 'implicitWidth' and 'implicitHeight' are specified

If I specify implicitWidth & implicitHeight for a popup, and open it in a window, and I resize the window down to ~80%, the popup scales correctly with the window. If I go down below ~70%, the popup starts to shrink faster than the window.

I made a minimal example here: https://github.com/DanielMcInnes/testDialog.git. If you resize the app window, note that the left side of the green popup remains in the same position relative to the numbers in the Text box, until you get down to ~70%.

If I specify 'width' and 'height' for the popup instead of 'implicitWidth' and 'implicitHeight', the problem goes away, the popup scales correctly as the window is resized.