Real question: You wrote "responsive-layout desktop app". What do you mean by this term?
From my experience, if you develop Qt widgets (like dialogs with many child widgets), it is possible (some might say easy) to make things resize correctly by using QHBoxLayout (horizontal) and QVBoxLayout (vertical).
I meant "Responsive" to mean something like media query breakpoints - at certain widths, Telegram will rearrange entire UI elements and change the navigation paradigm.
From my experience, if you develop Qt widgets (like dialogs with many child widgets), it is possible (some might say easy) to make things resize correctly by using QHBoxLayout (horizontal) and QVBoxLayout (vertical).