My first step was to investigate how collapsible elements work.

I spent some time looking at all of the collapsible element examples at:
https://www.mediawiki.org/wiki/Manual:Collapsible_elements/Demo/Advanced

the standard collapsible elements have predictable class changes based on state, but that isn't necessarily true of custom toggles. So, I came up with selectors that identify toggles based on class and identify expand/collapse state based on aria-expanded.

The remote collapsible example is problematic though, as it does not correctly handle the aria-expanded attribute: one of the toggles has aria-expanded=true even if the content is collapsed, and the other toggle doesn't use aria-expanded at all.
https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/Attributes/aria-expanded

I believe that's a bug that might need to be resolved separately

I also looked at the makeCollapsible module:
https://gerrit.wikimedia.org/r/plugins/gitiles/mediawiki/core/+/refs/heads/master/resources/src/jquery/jquery.makeCollapsible.js

I've come up with the following example code that expands/collapses elements toolbar links. It can be pasted directly into the browser javascript console. It has some limitations:

• it doesn't work on the aforementioned problematic example
• it currently fires the click event on collapsible elements, which is probably a nightmare for analytics. I need to figure out how to run the handler in makeCollapsible
• because I need to provide an anchor for portlet links, it currently brings the viewport back up to the top of the page on click.
• no i18n at the moment, since I've just hardcoded the label

const expandCollapsibleToolbarLinkId = 't-expand-all',
	collapseCollapsibleToolbarLinkId = 't-collapse-all';

function toggleCollapsible( expanded ) {
	$( `.mw-collapsible-toggle[aria-expanded="${expanded}"], .mw-customtoggle[aria-expanded="${expanded}"], [class^="mw-customtoggle-"][aria-expanded="${expanded}"]` ).each(
		function () {
		$( this ).trigger( 'click' );
		} );
}

mw.util.addPortletLink(
	'p-tb',
	'#',
	'expand all',
	expandCollapsibleToolbarLinkId
);

$( `#${expandCollapsibleToolbarLinkId}` ).on( 'click', function () {
	$( this ).hide();
	$( `#${collapseCollapsibleToolbarLinkId}` ).show();
	toggleCollapsible( false );
} );
mw.util.addPortletLink(
	'p-tb',
	'#',
	'collapse all',
	collapseCollapsibleToolbarLinkId
);

$( `#${collapseCollapsibleToolbarLinkId}` )
	.hide()
	.on( 'click', function () {
		$( this ).hide();
		$( `#${expandCollapsibleToolbarLinkId}` ).show();
		toggleCollapsible( true );
} );

My next step will be to see how I can avoid triggering click, as that's really just a hacky shortcut, IMO.

Change 961789 had a related patch set uploaded (by Jsn.sherman; author: Jsn.sherman):

[mediawiki/core@master] Add toolbox link to expand all collapsible elements on a page

https://gerrit.wikimedia.org/r/961789

Test wiki created on Patch demo by JSherman (WMF) using patch(es) linked to this task:
https://patchdemo.wmflabs.org/wikis/366e29eac4/w

Test wiki on Patch demo by JSherman (WMF) using patch(es) linked to this task was deleted:

https://patchdemo.wmflabs.org/wikis/366e29eac4/w/

Test wiki created on Patch demo by JSherman (WMF) using patch(es) linked to this task:
https://patchdemo.wmflabs.org/wikis/0dbf2d63a1/wiki/Manual:Collapsible_elements/Demo/Advanced

Test wiki created on Patch demo by JSherman (WMF) using patch(es) linked to this task:
https://patchdemo.wmflabs.org/wikis/d3edbcabb6/w

Test wiki on Patch demo by JSherman (WMF) using patch(es) linked to this task was deleted:

https://patchdemo.wmflabs.org/wikis/0dbf2d63a1/w/

Change 961789 merged by jenkins-bot:

[mediawiki/core@master] add portlet to toggle all collapsible elements

https://gerrit.wikimedia.org/r/961789

Test wiki on Patch demo by JSherman (WMF) using patch(es) linked to this task was deleted:

https://patchdemo.wmflabs.org/wikis/d3edbcabb6/w/

Change 1002613 had a related patch set uploaded (by Krinkle; author: Krinkle):

[mediawiki/core@master] mediawiki.page.ready: Move toggleAllCollapsibles.js file to this module

https://gerrit.wikimedia.org/r/1002613

Change 1002613 merged by jenkins-bot:

[mediawiki/core@master] mediawiki.page.ready: Move toggleAllCollapsibles.js file to this module

https://gerrit.wikimedia.org/r/1002613