Use a more modern / appropriate set of widgets for the "New comparison" empty state wizard
When you start the app by itself or when you click the "+" button, you get a UI layout like this:
The three buttons act as if they were tabs or switchers, but their appearance doesn't really convey that, and their arrangement in a 3x2 grid layout with the components below it makes for a somewhat rigid-feeling UI (without explanations, #560):
It would be nice if it could use a set of widgets that better communicates the flow. At first, looking at other apps and the gtk4-demo
app, I thought maybe it could use a set of GtkListBox with ">", within some sort of paging system, like GNOME Calendar uses in its "Manage Calendars" dialog to show the list of calendars vs their properties, but maybe that's a bit of an intrusive change, technically.
A probably simpler alternative that would be pretty similar to your current approach, but more notebook-like, could be to use GtkStack:
This widget takes care of the containing, crossfade animation etc. for you, and it exists both in GTK3 and in GTK4. I don't know if it's the perfect/ultimate solution, but I think it would already be an improvement, and probably easier to implement than other approaches (I'm guessing this is easy-enough for a newcomer to do even, so tagging as such).
This also opens up the opportunity for the widgets it contains to be laid out a bit differently: instead of 3 file/folderpicker buttons on a single row with 3 columns, they could maybe be in a vertical arrangement with some labels next to them to identify them, something like "First file:", "Second file:", "Third file (optional):" (you wouldn't need to bother with a checkbox then).
...and then since they wouldn't be so horizontally-constrained, you could at least have some 6 or 12px padding on the sides of the whole thing when the window width is not super wide.
Bonus points: this approach might have better accessibility overall, too. I'm far from being an expert on this, but I'd think the hierarchy/relationships between widgets would be clearer behind the scenes.