GtkBox applies spacing on cells that are not visible
Consider you have a GtkBox with three labels in it and a spacing of 5 pixels between each label. If you hide the middle label then two labels will be shown with a spacing of 10 pixels between them (the hidden label has size zero, but the spacing is still there). This doesn't seem to be useful - I would expect a hidden cell to act like it doesn't exist and there to continue to be 5 pixel spacing between the remaining elements.
I propose we add a flag added to ignore spacing for invisible elements - or perhaps this can be a behaviour change for GTK 4.
Edited by Robert Ancell