Expose QScrollArea as native widget

[dstansby]

Fixes https://github.com/napari/magicgui/issues/431 - this allows an external user to access the QScrollArea by returning it as the native widget. I think this is a sensible thing to do anyway, and it fixes a bug with the scroll bars not showing up when a widget is consumed by napari.

[codecov]

Codecov Report

Merging #429 (ebab119) into main (919acb2) will decrease coverage by 0.00%.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##             main     #429      +/-   ##
==========================================
- Coverage   88.99%   88.99%   -0.01%     
==========================================
  Files          30       30              
  Lines        3935     3944       +9     
==========================================
+ Hits         3502     3510       +8     
- Misses        433      434       +1     

Impacted Files	Coverage Δ
magicgui/backends/_qtpy/widgets.py	86.95% <75.00%> (-0.06%)	⬇️
magicgui/widgets/_bases/widget.py	87.91% <100.00%> (+0.20%)	⬆️
magicgui/widgets/_protocols.py	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 919acb2...ebab119. Read the comment docs.

[tlambert03]

thank you @dstansby!

I am slightly concerned about this (but I'm not sure, and could be convinced otherwise). The main reason is that there is sort of an implicit API that when you do widget.native on a container, you get the widget that has the layout... and I know a few people are accessing that to do things like widget.native.layout().addStretch()or something... I _think_ this would break that... and would require everyone consumingwidget.nativeto add anif isinstance()` check to see if the widget is a QScrollArea or not if they wanted to avoid potential errors. I know this fixes the scrollbars in napari, but I think it might create errors elsewhere. thoughts?

[dstansby]

That all makes sense, and I think you're repeating yourself from the original PR, so sorry about that. So, it seems we should keep .native as the widget that has the layout (and document that if it isn't already, I'll check that), and expose another attribute that 3rd party libraries use for embedding. Since the widget with the layout is a child of the scroll area, there's no way around needing to expose the scroll area for embedding for scrolling to work.

[dstansby]

I've added a new property, root_native_widget, which is designed to be whathever the top widget is and is designed to be embedded by third parties. Not sure if this a good approach, and the API might need bikeshedding, but thought I'd push some code for discussion 😃

[tlambert03]

I think i like it! will have a play shortly... but this seems like a very good solution

[tlambert03]

looks excellent, thank you @dstansby!