[Paper] Reduce the default elevation

[oliviertassinari]

This is a proposal. Google seems to use fewer and fewer elevation with its product design. I think that we can reflect this trend by decreasing the default elevation of the Paper from 2 to 1. Please review!

Breaking change

Change the default Paper elevation to match the Card and the Expansion Panel:

-<Paper />
+<Paper elevation={2} />

[mui-pr-bot]

Details of bundle changes.

Comparing: 1fa8f94...30b70e1

bundle	parsed diff	gzip diff	prev parsed	current parsed	prev gzip	current gzip
@material-ui/core	-0.00%	-0.01%	347,568	347,556	89,893	89,888
@material-ui/core/Paper	0.00%	0.00%	68,404	68,404	20,053	20,053
@material-ui/core/Paper.esm	0.00%	0.00%	60,735	60,735	18,785	18,785
@material-ui/core/Popper	0.00%	0.00%	34,930	34,930	11,917	11,917
@material-ui/core/styles/createMuiTheme	0.00%	0.00%	17,877	17,877	5,962	5,962
@material-ui/core/useMediaQuery	0.00%	0.00%	2,463	2,463	1,044	1,044
@material-ui/lab	0.00%	0.00%	148,119	148,119	43,913	43,913
@material-ui/styles	0.00%	0.00%	53,143	53,143	15,443	15,443
@material-ui/system	0.00%	0.00%	17,132	17,132	4,519	4,519
Button	0.00%	0.00%	88,669	88,669	26,423	26,423
Modal	0.00%	0.00%	82,688	82,688	24,802	24,802
colorManipulator	0.00%	0.00%	3,745	3,745	1,537	1,537
docs.landing	0.00%	0.00%	49,820	49,820	10,807	10,807
docs.main	0.00%	0.00%	644,630	644,630	200,877	200,877
packages/material-ui/build/umd/material-ui.production.min.js	-0.00%	-0.01%	296,257	296,245	82,811	82,806

Generated by 🚫 dangerJS against 30b70e1

[eps1lon]

I'd be careful with these changes since they're already easily customizable. Elevation is also a relative to one another so I'm not sure about all the cascading effects this change might have.

That being said standard sheets in the spec actually only have 0 resting elevation. Cards have 1.

Since Paper is an ambiguous term I would rather keep this default value.

[oliviertassinari]

@eps1lon I like the idea of the Card and the Paper having the same default elevation:

Elevation 2 (Paper)

Elevation 1 (Card)

In the future, I think that it would be great to have a way to get an outlined Paper:

Google Keep

[oliviertassinari]

@eps1lon I understand your concern. I think that we should move forward. The motivation is to reduce the default shadows. I think that we should discourage the usage of too many elevations. It works best on mobile than on desktop. After this change, the Card, the Paper and the Expansion panel use the same default value. That's better. It's also closer to chrome://settings/.