Qt Widgets maintainers: which screen is hardest to modernize?

I’m building Fluent-Qt, but I’m stuck on a prioritization problem. I can keep adding controls, yet a longer component list does not tell me whether the library helps someone maintain a real desktop app.

The case I care about is incremental adoption: modernizing one existing screen without replacing Qt’s object model, event loop, or CMake setup, and without turning it into a full QML migration.

If you maintain a Qt Widgets app, which single screen or interaction is the hardest to modernize today? What specifically makes it difficult in your codebase?

I’m thinking about dense data views, navigation, overlays and dialogs, or window chrome—but I may be missing the real blocker.

For context, here is the WebAssembly Gallery for the approach I’m testing:
https://calvinhxx.github.io/Flue...

I’ll use the replies to pick one screen for a documented integration case study and publish the before/after diff, time spent, and platform limitations.