I have a Container that holds another Container, and a Form Panel, using HBOX layout. Flex can be set on the chlid Container, but not on the form panel, via Designer. If I add "flex: 1" to the code manually, it works as expected. In general, Flex seems to have this problem -- it is not always there when you would expect it, and instead only the "MaintainFlex" property is available.
I used "duplicate" to copy everything and start paring it down to a minimal example, and the duplicated version works (while the original still doesn't). So, I have two layouts, exactly the same, and one panel has flex available and one doesn't. Very strange. I'll still see if I can distill what the difference is.
No luck. Saving it as a new file and then reopening it fixed the problem also. Maybe data corruption? Here is the file anyway. The "notes" panel had no flex property (across many saves and reopens - but saving under a new name seems to be different).