I want the labels to be on the left, and the widgets on the right. So, I
use a flex on the label, so it pushes widgets to the right, whatever the
window width is. And widgets has no flex, so they use their minimum size.
But without flex, the slider has a zero width! And with a flex, it resizes
as we move the cursor.
BTW, with your example, when the slider cursor is on its minimal value
(left), it partially hides the label :o/