After successfully setting up Ubuntu 16 with Window Maker I was able to recheck your test app. It behaves correctly under Window Maker:
openapp ./resize.app &> A.out.
2018-05-02 12:05:04.413 resize[3378:3378] styleoffsets ... guessing offsets
2018-05-02 12:05:04.414 resize[3378:3378] styleoffsets ... guessing offsets
2018-05-02 12:05:04.529 resize[3378:3378] Content view set frame to {x = 1; y = 9; width = 140; height = 140}
Now resizing the window ...
2018-05-02 12:05:08.426 resize[3378:3378] Content view set frame to {x = 1; y = 9; width = 149; height = 140}
2018-05-02 12:05:08.426 resize[3378:3378] Content view set frame to {x = 1; y = 9; width = 149; height = 140}
Looks good!
So the issue is somehow related to this Ubuntu ubity stuff. No problem for me as we want to and can run Window Maker anyway.