Do you remember a popular QT app that would turn a row of elements into a column of elements when shrinking a window, though ?

Was it also common to make sure your Qt app was working fine on the small portrait resolution of the mobile computers that were going to be invented any decade now ?

Flex is a layout system (a fairly good one, that I wish had existed on the web since the start. Rows, columns, wrap. Good.)

; responsive is a design considération (what do I as a human being decide should be visible when the screen is just too small to show everything.)

My claim is that "responsive" as a design issue is deeply linked to mobile computers, so it's not a surprise that it was not a big conxern before.