QT allows decorations to be disabled and so works well with a tiling compositor. GTK doesn't. There are various GTK patches and hacks floating around to address this but nothing upstream. Older software generally works well by virtue of pre-dating client-side decorations.
GTK 3 has been downhill usability-wise for me. One of the primary reasons I've stopped as a developer to use it entirely.
Also, the path bar is long gone. It used to support tab completion, which was actually faster than selecting files and/or searching for them. I don't know what they're thinking.
The list of gripes I have with GTK 3 is increasing at each release.
Ordinarily QT will automatically render decorations inside an application's Wayland window, but if the QT_WAYLAND_DISABLE_WINDOWDECORATION environment variable is set, it only renders the content inside the decorations. The window manager can them draw it's own decorations around the outside, when it composites the desktop.
It's not ideal. There's no mechanism for an application to supply hints to the window manager to control it's decorations like in X. Nor can the application draw custom borders like when using client-side decorations. There's really no other way of implementing the stacking and tabbed window layouts which are popular in tiling window managers though. Thankfully it's good enough for most applications and the compositor can have overrides (e.g in a user config file) for special cases.