thanks for your input! It´s a good idea to define breakpoints where widgets stretch to container, each of the breakpoints with different stretch percentages. I didn´t think about that.
However, your example is useful when the content of the columns is predictable, for example if the content of each column is equally long. I have columns that can be filled by the user.
I just found that pasting the CSS code “min-width: min-content” in the CSS box brings me a step forward. Still, the breakpoints also depend on the minimum width: If I have two boxes that already fill 45% of the screen, the stretch percentage of the breakpoint should be higher, or I would have a blank space.
If I don´t know that percentage, and enter a number that is too high, I will lose compactness, which was my first goal.
The second one was to fill up the remaining space, just for the sake of beauty.
I guess I can live without achieving the second goal, and I learned something new.