It is as follows:
Pic 3 with these long spacers contains complex widgets MechPartWidget presented in pic 4. You can see a tiny selection above the 2nd vertical spacer
Then, in MechPartWidget there are no layouts/widgets with vertical horizontal policy set to expanding.
Next, in the MechPartWidget there is a custom widget InventoryWidget. I tried to add QVBoxLayout to it, several widgets, and then a vertical spacer - and I expect this own-added layout would be honoured only inside boundaries of InventoryWidget, yet it isn't.