Layout.fillWidth sometimes has no effect

in the 6.8 branch today's checkout (other people seem to encounter this in 6.7 branch as well), sometimes the attached property Layout.fillWidth has no effect.

In the attached example the item "body" has a Layout.preferredWidth of 200 and fillWidth: true

however its width always stays 200 even if fillWidth is true and resizing the window won't have any effect. the green rectangle on top which has just fillWidth but no other hints will resize correctly, body and all its containing layout won't