What I did

• Introduce a new addon type: page (prefix with experimental for now)
• Simplify the API for this new addon type
• Make API for addon injection stricter
• Fix a bug where selection during preview initialization was ignored
• Make the default route / even if there's no query
• Refactor the layout to hide the storybook main area when the route doesn't match (as apposed to the exception /settings was before)

How to test

• create a sandbox
• add a custom manager entry
• here's the code I've been using to test with:

  import type { ComponentProps, FC } from 'react';
  import React, { memo, Fragment } from 'react';
  import { addons, types } from '@storybook/manager-api';
  
  import { Icons, IconButton, Bar, TabButton } from '@storybook/components';
  import { styled } from '@storybook/theming';
  import { Route } from '@storybook/router';
  
  const Toolbar = styled(
    ({ shown = true, ...props }: ComponentProps<typeof Bar> & { shown: boolean }) => (
      <Bar {...props} />
    )
  )(
    {
      position: 'absolute',
      left: 0,
      right: 0,
      top: 0,
      transition: 'transform .2s linear',
    },
    ({ shown }) => ({
      transform: shown ? 'translateY(0px)' : 'translateY(-40px)',
    })
  );
  
  const FrameWrap = styled.div<{ offset: number }>(({ offset }) => ({
    position: 'absolute',
    overflow: 'auto',
    left: 0,
    right: 0,
    bottom: 0,
    top: offset,
    zIndex: 3,
    transition: 'all 0.1s linear',
    height: `calc(100% - ${offset}px)`,
    background: 'transparent',
  }));
  
  const Centered = styled.div({
    position: 'relative',
  
    width: '100%',
    height: '100%',
    display: 'flex',
    alignItems: 'center',
    justifyContent: 'center',
  });
  
  addons.register('@storybook/addon-debugger', (api) => {
    addons.add('@storybook/addon-debugger/tool', {
      title: 'debugger!',
      type: types.TOOLEXTRA,
      match: ({ viewMode }) => !!(viewMode && viewMode.match(/^(story|docs)$/)),
      render: () => (
        <Fragment>
          <IconButton
            key="debugger"
            title="navigate to debugger-view"
            onClick={() =>
              api.navigateUrl(`/debugger/${api.getCurrentStoryData().id}`, { plain: false })
            }
          >
            <Icons icon="lightning" />
          </IconButton>
        </Fragment>
      ),
    });
  
    const DebuggerPage: FC = () => {
      return (
        <Route path="/debugger" startsWith>
          <Toolbar shown border>
            <TabButton active>A tab</TabButton>
            <IconButton
              key="first"
              title="Go to first story, for some reason. It's just a demo."
              onClick={() => api.selectFirstStory()}
            >
              <Icons icon="star" />
            </IconButton>
          </Toolbar>
          <FrameWrap offset={40}>
            <Centered>
              This is the contents of my addon, in a full viewport experience, what a joy!
            </Centered>
          </FrameWrap>
        </Route>
      );
    };
  
    addons.add('@storybook/addon-debugger/panel', {
      title: 'Debugger',
      type: types.experimental_PAGE,
      url: '/debugger',
      render: memo(DebuggerPage),
    });
  });

• navigate to /: expect the story to be shown
• navigate to /settings/about : expect the about page to be shown
• navigate to /debugger : expect the debugger panel to be shown
• click on the ⭐ icon in the toolbar (whilst on the debugger page : expect the route to change, and the first story to be shown
• use the browser back button : expect the debugger page to be shown again
• use the browser forward button : expect the story to be shown again
• click the ⚡ icon on the right side of the toolbar (whilst on the canvas) : expect the route to change and the debugger page to be shown

I have done the best I can to have some of this also make sense for the mobile view, but this is really up for grabs. We need to determine what to do regarding pages for mobile.

Checklist

• Make sure your changes are tested (stories and/or unit, integration, or end-to-end tests)
• Make sure to add/update documentation regarding your changes
• If you are deprecating/removing a feature, make sure to update
  MIGRATION.MD

Maintainers

• If this PR should be tested against many or all sandboxes,
  make sure to add the ci:merged or ci:daily GH label to it.
• Make sure this PR contains one of the labels below.

["cleanup", "BREAKING CHANGE", "feature request", "bug", "documentation", "maintenance", "dependencies", "other"]