Layout.preferredWidth is ignored

In my opinion, black and red rectangles (bug.png) should divide the space equally.

Here is the code with all the comments. Making troubleMaker invisible "solves" the issue.

P.S. Ok, it seems copy/paste from Qt Creator is not supported here at all. So I'll just attach the code instead of pasting it here. There you'll be able to find all the descriptions about the issue (maybe not an issue but a lack of my knowledge by the way..).