[EuiHeader] Update to set a new global --euiFixedHeadersOffset CSS variable

src-docs/src/components/guide_page/_guide_page.scss

@@ -1,22 +1,5 @@
 @import '../../../../src/global_styling/mixins/helpers';
 
-@include euiHeaderAffordForFixed;
-
-.guideBody {
-  // Override euiHeaderAffordForFixed mixin since the page template handles this now
-  padding-top: 0 !important; // stylelint-disable-line declaration-no-important
-}
-
-.euiBody--headerIsFixed--double {
-  @include euiHeaderAffordForFixed($euiHeaderHeightCompensation * 2);
-
-  .euiHeader:not([data-fixed-header]) {
-    // Force headers below the fullscreen.
-    // This shouldn't be necessary in consuming applications because headers should always be at the top of the page
-    z-index: 0;
-  }
-}
-
 .guideSideNav {
   @include euiSideNavEmbellish;
 }

src-docs/src/components/guide_page/guide_page_chrome.js

@@ -69,7 +69,7 @@ export class GuidePageChrome extends Component {
   scrollNavSectionIntoView = () => {
     // wait a bit for react to blow away and re-create the DOM
     // then scroll the selected nav section into view
-    requestAnimationFrame(() => {
+    setTimeout(() => {
       const sideNav = document.querySelector('.guideSideNav__content');
       const isMobile = sideNav?.querySelector('.euiSideNav__mobileToggle');
 

src-docs/src/views/header/header_elastic_pattern.tsx

@@ -1,4 +1,4 @@
-import React, { useState, useEffect } from 'react';
+import React, { useState } from 'react';
 import { Link } from 'react-router-dom';
 
 import {
@@ -53,14 +53,6 @@ export default () => {
     prefix: 'guideHeaderDeploymentPopover',
   });
 
-  useEffect(() => {
-    document.body.classList.add('euiBody--headerIsFixed--double');
-
-    return () => {
-      document.body.classList.remove('euiBody--headerIsFixed--double');
-    };
-  }, []);
-
   /**
    * Collapsible Nav
    */

src-docs/src/views/header/header_example.js

@@ -299,23 +299,34 @@ export const HeaderExample = {
           <p>
             Most consumers need a header that does not scroll away with the page
             contents. You can apply this display by applying the property{' '}
-            <EuiCode language="ts">{'position="fixed"'}</EuiCode>. This will
-            also add a class of <EuiCode>.euiBody--headerIsFixed</EuiCode> to
-            the window body.
+            <EuiCode language="ts">{'position="fixed"'}</EuiCode>. Multiple
+            fixed headers will automatically stack underneath one another
+            without manual positioning required.
           </p>
           <p>
-            You will then need to apply your own padding to this body class to
-            afford for the header height. EUI supplies a helper mixin that also
-            accounts for this height in flyouts and the collapsible nav. Simply
-            add{' '}
-            <EuiCode language="sass">@include euiHeaderAffordForFixed;</EuiCode>{' '}
-            anywhere in your SASS.
+            If you're using{' '}
+            <Link to="/templates/page-template">
+              <strong>EuiPageTemplate</strong>
+            </Link>
+            , a padding top will be automatically set based on the number of
+            fixed headers on the page.{' '}
+            <Link to="/layout/flyout">
+              <strong>EuiFlyouts</strong>
+            </Link>{' '}
+            will also automatically account for and sit below fixed headers.
+          </p>
+          <p>
+            If you're using your own custom layout, or have custom UI that needs
+            to sit below your fixed header(s), EUI provides a global CSS{' '}
+            <EuiCode language="css">var(--euiFixedHeadersOffset)</EuiCode>{' '}
+            variable. You can use this variable anywhere, or even override it,
+            to correctly offset any and all fixed header heights.
           </p>
         </>
       ),
       snippet: [
         '<EuiHeader position="fixed" />',
-        '@include euiHeaderAffordForFixed;',
+        'body { padding-block-start: var(--euiFixedHeadersOffset, 0) }',
       ],
       demo: <HeaderPosition />,
       demoPanelProps: {
@@ -446,17 +457,12 @@ export const HeaderExample = {
       text: (
         <p>
           Stacking multiple headers provides a great way to separate global
-          navigation concerns. However, the{' '}
-          <EuiCode language="ts">{'position="fixed"'}</EuiCode> option will not
-          be aware of the number of headers. If you do need fixed{' '}
-          <strong>and</strong> stacked headers, you will need to apply the SASS
-          helper mixin and pass in the correct height to afford for.
+          navigation concerns.
         </p>
       ),
       snippet: [
         `<EuiHeader theme="dark" position="fixed" />
 <EuiHeader position="fixed" />`,
-        '@include euiHeaderAffordForFixed($euiHeaderHeightCompensation * 2);',
       ],
       demo: <HeaderStacked />,
       demoPanelProps: {

src-docs/src/views/header/header_stacked.tsx

@@ -1,4 +1,4 @@
-import React, { useState, useEffect } from 'react';
+import React, { useState } from 'react';
 
 import {
   EuiBreadcrumb,
@@ -27,14 +27,6 @@ export default () => {
     },
   ];
 
-  useEffect(() => {
-    if (isFixed) document.body.classList.add('euiBody--headerIsFixed--double');
-
-    return () => {
-      document.body.classList.remove('euiBody--headerIsFixed--double');
-    };
-  }, [isFixed]);
-
   const headers = (
     <>
       <EuiHeader

src/components/collapsible_nav_beta/collapsible_nav_beta.stories.tsx

@@ -404,8 +404,8 @@ export const MultipleFixedHeaders: Story = {
       <EuiHeader position="fixed">
         <EuiHeaderSection>
           <EuiCollapsibleNavBeta {...args}>
-            This story tests that EuiCollapsibleNav's fixed header detection &
-            offsetting works as expected
+            This story tests that EuiCollapsibleNav automatically adjusts its
+            position & height for multiple fixed headers
           </EuiCollapsibleNavBeta>
           Second header
         </EuiHeaderSection>

src/components/collapsible_nav_beta/collapsible_nav_beta.styles.ts

@@ -10,18 +10,30 @@ import { css } from '@emotion/react';
 import { UseEuiTheme } from '../../services';
 import { logicalCSS, euiYScroll } from '../../global_styling';
 import { euiShadowFlat } from '../../themes';
+import { euiHeaderVariables } from '../header/header.styles';
 
 export const euiCollapsibleNavBetaStyles = (euiThemeContext: UseEuiTheme) => {
   const { euiTheme } = euiThemeContext;
 
+  // At least for serverless, EuiCollapsibleNav is only going to be used with 1
+  // fixed header. For those scenarios, we can prevent a minor layout jump on
+  // page load by setting the CSS var fallback to the height of a single header
+  const defaultHeaderHeight = euiHeaderVariables(euiThemeContext).height;
+  const fixedHeaderOffset = `var(--euiFixedHeadersOffset, ${defaultHeaderHeight})`;
+
   return {
     euiCollapsibleNavBeta: css`
+      /* Fixed header affordance */
+      ${logicalCSS('top', fixedHeaderOffset)}
+      ${logicalCSS('height', `calc(100% - ${fixedHeaderOffset})`)}
+
       /* This extra padding is needed for EuiPopovers to have enough
          space to render with the right anchorPosition */
       ${logicalCSS('padding-bottom', euiTheme.size.xs)}
 
-      /* Allow the nav to scroll, in case consumers don't use EuiFlyoutBody/EuiFyoutFooter */
-      ${euiYScroll(euiThemeContext)}
+      /* Allow the nav to scroll, in case consumers don't use EuiFlyoutBody/EuiFyoutFooter
+         Height is already set by header affordance above */
+      ${euiYScroll(euiThemeContext, { height: false })}
 
       /* In case things get really dire responsively, ensure the footer doesn't overtake the body */
       .euiFlyoutBody {

src/components/collapsible_nav_beta/collapsible_nav_beta.test.tsx

@@ -12,8 +12,6 @@ import { render } from '../../test/rtl';
 import { shouldRenderCustomStyles } from '../../test/internal';
 import { requiredProps } from '../../test';
 
-import { EuiHeader } from '../header';
-
 import { EuiCollapsibleNavBeta } from './collapsible_nav_beta';
 
 describe('EuiCollapsibleNavBeta', () => {
@@ -66,20 +64,6 @@ describe('EuiCollapsibleNavBeta', () => {
     expect(onCollapseToggle).toHaveBeenLastCalledWith(true);
   });
 
-  it('automatically accounts for fixed EuiHeaders in its positioning', () => {
-    const { getByTestSubject } = render(
-      <EuiHeader position="fixed">
-        <EuiCollapsibleNavBeta data-test-subj="nav">
-          Nav content
-        </EuiCollapsibleNavBeta>
-      </EuiHeader>
-    );
-    expect(getByTestSubject('nav')).toHaveStyle({
-      'inset-block-start': '48px',
-      'block-size': 'calc(100% - 48px)',
-    });
-  });
-
   describe('responsive behavior', () => {
     const mockWindowResize = (width: number) => {
       window.innerWidth = width;

src/components/collapsible_nav_beta/collapsible_nav_beta.tsx

@@ -19,7 +19,6 @@ import React, {
 import classNames from 'classnames';
 
 import { useEuiTheme, useGeneratedHtmlId, throttle } from '../../services';
-import { mathWithUnits, logicalStyle } from '../../global_styling';
 
 import { CommonProps } from '../common';
 import { EuiFlyout, EuiFlyoutProps } from '../flyout';
@@ -83,7 +82,6 @@ export const EuiCollapsibleNavBeta: FunctionComponent<
   id,
   children,
   className,
-  style,
   initialIsCollapsed = false,
   onCollapseToggle,
   width: _width = 248,
@@ -144,32 +142,6 @@ export const EuiCollapsibleNavBeta: FunctionComponent<
     return _width;
   }, [_width, isOverlayFullWidth, isPush, isCollapsed, headerHeight]);
 
-  /**
-   * Header affordance
-   */
-  const [fixedHeadersCount, setFixedHeadersCount] = useState<number | false>(
-    false
-  );
-  useEffect(() => {
-    setFixedHeadersCount(
-      document.querySelectorAll('.euiHeader[data-fixed-header]').length
-    );
-  }, []);
-
-  const stylesWithHeaderOffset = useMemo(() => {
-    if (!fixedHeadersCount) return style;
-
-    const headersOffset = mathWithUnits(
-      headerHeight,
-      (x) => x * fixedHeadersCount
-    );
-    return {
-      ...style,
-      ...logicalStyle('top', headersOffset),
-      ...logicalStyle('height', `calc(100% - ${headersOffset})`),
-    };
-  }, [fixedHeadersCount, style, headerHeight]);
-
   /**
    * Prop setup
    */
@@ -205,15 +177,13 @@ export const EuiCollapsibleNavBeta: FunctionComponent<
     isOverlayFullWidth && styles.isOverlayFullWidth,
   ];
 
-  // Wait for any fixed headers to be queried before rendering (prevents position jumping)
-  const flyout = fixedHeadersCount !== false && (
+  const flyout = (
     <EuiFlyout
       aria-label={defaultAriaLabel}
       {...rest} // EuiCollapsibleNav is significantly less permissive than EuiFlyout
       id={flyoutID}
       css={cssStyles}
       className={classes}
-      style={stylesWithHeaderOffset}
       size={width}
       side={side}
       focusTrapProps={focusTrapProps}

src/components/datagrid/_data_grid.scss

@@ -49,12 +49,23 @@
 }
 
 // This is used to remove extra scrollbars on the body when fullscreen is enabled
+// and tweak any components that account for fixed headers
 .euiDataGrid__restrictBody {
   height: 100vh;
   overflow: hidden;
 
-  .euiHeader {
-    z-index: $euiZHeaderBelowDataGrid;
+  .euiHeader[data-fixed-header] {
+    // !important needed to override header inline styles
+    z-index: $euiZHeaderBelowDataGrid !important; // stylelint-disable-line declaration-no-important
+  }
+
+  .euiOverlayMask[data-relative-to-header='below'] {
+    top: 0;
+  }
+
+  .euiFlyout {
+    top: 0;
+    height: 100%;
   }
 }
 

src/components/flyout/__snapshots__/flyout.test.tsx.snap

@@ -1086,12 +1086,12 @@ Array [
 exports[`EuiFlyout renders extra screen reader instructions when fixed EuiHeaders headers exist on the page 1`] = `
 <body
   class="euiBody--headerIsFixed euiBody--hasFlyout"
-  data-fixed-headers="1"
 >
   <div>
     <div
       class="euiHeader emotion-euiHeader-fixed-default"
       data-fixed-header="true"
+      style="inset-block-start: 0px;"
     />
     <div>
       <div

src/components/flyout/flyout.styles.ts

@@ -98,9 +98,9 @@ export const euiFlyoutStyles = (euiThemeContext: UseEuiTheme) => {
   return {
     euiFlyout: css`
       position: fixed;
-      ${logicalCSS('top', 0)}
       ${logicalCSS('bottom', 0)}
-      ${logicalCSS('height', '100%')}
+      ${logicalCSS('top', 'var(--euiFixedHeadersOffset, 0)')}
+      ${logicalCSS('height', 'calc(100% - var(--euiFixedHeadersOffset, 0))')}
       z-index: ${euiTheme.levels.flyout};
       background: ${euiTheme.colors.emptyShade};
       display: flex;

src/components/header/__snapshots__/header.test.tsx.snap

@@ -28,6 +28,7 @@ exports[`EuiHeader renders in fixed position 1`] = `
 <div
   class="euiHeader emotion-euiHeader-fixed-default"
   data-fixed-header="true"
+  style="inset-block-start: 0px;"
 >
   <span>
     Hello!