[Accordion] Horizontal padding

[esseswann]

• The issue is present in the latest release.
• I have searched the issues of this repository and believe that this is not a duplicate.

Current Behavior 😯

Expansion panel has 24px padding, while list items have 16px. I am not sure if it corresponds to the spec, but it makes you add extra elements for unification

Expected Behavior 🤔

Elements should have unified horizontal padding

Your Environment 🌎

Tech	Version
Material-UI	v4.9.9
React	16.13.1
Browser	Chrome 80.0.3987.163

[oliviertassinari]

@esseswann The Material Design specification does no longer covers the expansion panel component. This gives us more freedom around the actual look.

Regarding your request, I have looked at Bootstrap, they use the style of the Card Header. So in our case, I think that it would make sense to do the same and to move forward with your request.

diff --git a/packages/material-ui/src/ExpansionPanelSummary/ExpansionPanelSummary.js b/packages/material-ui/src/ExpansionPanelSummary/ExpansionPanelSummary.js
index 7ed48db1a..b61281403 100644
--- a/packages/material-ui/src/ExpansionPanelSummary/ExpansionPanelSummary.js
+++ b/packages/material-ui/src/ExpansionPanelSummary/ExpansionPanelSummary.js
@@ -17,7 +17,7 @@ export const styles = (theme) => {
       display: 'flex',
       minHeight: 8 * 6,
       transition: theme.transitions.create(['min-height', 'background-color'], transition),
-      padding: theme.spacing(0, 3),
+      padding: theme.spacing(0, 2),
       '&:hover:not($disabled)': {
         cursor: 'pointer',
       },

Do you want to submit a pull request? :)

---

On a side note, I think that we should rename the component in v5 ExpansionPanel -> Accordion. The pros:

• Help users migrating from Bootstrap to Material-UI: https://getbootstrap.com/docs/4.4/components/collapse/#accordion-example.
• Help users already familiar with a11y components: https://www.w3.org/TR/wai-aria-practices-1.1/examples/accordion/accordion.html.
• Break free from MD, help with future themes
• The query has, at least, x5 more SEO traffic.

[esseswann]

Do you want to submit a pull request? :)

Can do, should it be from some v5 branch or 4.x?

[oliviertassinari]

We don't have a v5 branch yet. master is perfect.