@tomendop see documentation for "Layout QML Type". It states:

Note: It is not recommended to have bindings to the x, y, width, or height properties of items in a layout, since this would conflict with the goals of Layout, and can also cause binding loops.

I can't say exactly what is going wrong in your example, but it is basically that you are doing something you aren't supposed to do when you choose to use the Layout mechanism.

I think it can be quite confusing as QML has a number of different ways to manage layout and it isn't always obvious which way is best or most appropriate.

@Jkimmy Thank you for your help.

@A-A-SEZEN said in QTableWidget's width behaves strangely when applying any CSS:

Check QTableWidget font, before css and after.

Just changed the font to Consolas.
Anyways, adding

ui->tableWidget_memory->horizontalHeader()->resizeSections(QHeaderView::ResizeMode::ResizeToContents);

to my code solved this problem.

@McLion You can use layouts for those labels

Besides what @Rondog mentions, you could also read about layouts
http://doc.qt.io/qt-5.5/designer-layouts.html

If you assign a layout to the window, and put the tab in it, Qt can then
automatically adjust the size if user resize the window.
It takes a little bit to learn layouts but the reward is great.