On the contrary, Gtk and libhandy have excellent usability on phones and make applications that can run seamlessly on phones and desktops.
> Flutter, Electron, React Native
I'm not touching any of these with a stick. There no justification for demanding to run a web browser when a native application would do.
Show me a single app that uses transitions and fading in/out sidebars, and has swipe gesture integrations.
As long as this is not supported by concept (Gtk boxes will never allow this type of scene graph) I don't think Gtk can be called mobile-ready.
Mobile screen space is very limited, and as long as Gtk is static as a scene graph by nature, you'll have to do thousands of hacks to "somehow" fake it. The gsettings app on phosh is a damn dirty hack of hacks.
And with pure CSS, you would need literally less than 10 lines of layouting code.
I'm not saying electron is better, but Gtk as a layouting concept is heavily outdated. If you don't think so: read the phosh codebase. You'll start to cry after an hour, how devs persist to make this somehow work due to beliefs that Gtk is made for this. It isn't.
You're really overestimating the Pinephone's hardware. Firefox takes about 10 seconds to open.