Anchor tabbed modal using only CSS (#237431)

## Summary


This PR switches the implementation to anchor the tabbed modals to a
pure CSS solution, this solution had been suggested
[previously](#189399 (review))
however at the time it wasn't apparent we could leverage the `Global`
component from emotion, to coordinate insert and removing styles global
styles.


https://github.com/user-attachments/assets/cb67afea-a63f-40d1-9789-60ea3d99c363

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Author: eokoneyo

src/platform/packages/shared/shared-ux/modal/tabbed/src/tabbed_modal.test.tsx

@@ -16,49 +16,73 @@ import { TabbedModal, type IModalTabDeclaration } from './tabbed_modal';
 describe('TabbedModal', () => {
   const modalOnCloseHandler = jest.fn();
 
+  interface TabState {
+    inputText: string;
+  }
+
+  const getTabDefinition = (
+    actionHandlerFn: (args: { state: TabState }) => void
+  ): IModalTabDeclaration<TabState> => ({
+    id: 'logUserInput',
+    name: 'log user input',
+    reducer(state = { inputText: '' }, action) {
+      switch (action.type) {
+        case 'UPDATE_TEXT_VALUE':
+          return {
+            ...state,
+            inputText: action.payload,
+          };
+        default:
+          return state;
+      }
+    },
+    content: ({ state, dispatch }) => {
+      const onChange = (e: { target: { value: any } }) => {
+        dispatch({ type: 'UPDATE_TEXT_VALUE', payload: e.target.value });
+      };
+
+      return (
+        <EuiFieldText
+          data-test-subj="log-user-input-field"
+          placeholder="Placeholder text"
+          value={state.inputText}
+          onChange={onChange}
+          aria-label="Use aria labels when no actual label is in use"
+        />
+      );
+    },
+    modalActionBtn: {
+      id: 'logUserMessage',
+      dataTestSubj: 'log-user-message',
+      label: 'log user message',
+      handler: actionHandlerFn,
+    },
+  });
+
+  it('renders correctly', () => {
+    const tabDefinition = getTabDefinition(jest.fn());
+
+    render(
+      <TabbedModal
+        tabs={[tabDefinition]}
+        defaultSelectedTabId="logUserInput"
+        onClose={modalOnCloseHandler}
+      />
+    );
+
+    // we assert this value exists because we target this className to override some styles
+    const overlayMask = document.querySelector('.euiOverlayMask');
+
+    expect(overlayMask).toBeInTheDocument();
+    expect(overlayMask?.querySelector('[id^="tabbedModal"]')).toBeInTheDocument();
+  });
+
   describe('modal configuration', () => {
     const mockedHandlerFn = jest.fn();
 
-    const tabDefinition: IModalTabDeclaration<{
-      inputText: string;
-    }> = {
-      id: 'logUserInput',
-      name: 'log user input',
-      reducer(state = { inputText: '' }, action) {
-        switch (action.type) {
-          case 'UPDATE_TEXT_VALUE':
-            return {
-              ...state,
-              inputText: action.payload,
-            };
-          default:
-            return state;
-        }
-      },
-      content: ({ state, dispatch }) => {
-        const onChange = (e: { target: { value: any } }) => {
-          dispatch({ type: 'UPDATE_TEXT_VALUE', payload: e.target.value });
-        };
-
-        return (
-          <EuiFieldText
-            data-test-subj="log-user-input-field"
-            placeholder="Placeholder text"
-            value={state.inputText}
-            onChange={onChange}
-            aria-label="Use aria labels when no actual label is in use"
-          />
-        );
-      },
-      modalActionBtn: {
-        id: 'logUserMessage',
-        dataTestSubj: 'log-user-message',
-        label: 'log user message',
-        handler: mockedHandlerFn,
-      },
-    };
-
     it("when a single tab definition is passed it simply renders it's content into the modal component without tabs", async () => {
+      const tabDefinition = getTabDefinition(mockedHandlerFn);
+
       render(
         <TabbedModal
           tabs={[tabDefinition]}
@@ -77,6 +101,8 @@ describe('TabbedModal', () => {
     });
 
     it('renders the tabbed modal with tabs for tab definition with length greater than 1', async () => {
+      const tabDefinition = getTabDefinition(mockedHandlerFn);
+
       render(
         <TabbedModal
           tabs={[tabDefinition, { ...tabDefinition, id: 'anotherTab', name: 'another tab' }]}

src/platform/packages/shared/shared-ux/modal/tabbed/src/tabbed_modal.tsx

@@ -8,16 +8,14 @@
  */
 
 import React, {
-  useRef,
   useMemo,
-  useState,
-  useLayoutEffect,
   useCallback,
   Fragment,
   type ComponentProps,
   type FC,
   type ReactElement,
 } from 'react';
+import { Global } from '@emotion/react';
 import {
   EuiButton,
   EuiModal,
@@ -77,38 +75,9 @@ const TabbedModalInner: FC<ITabbedModalInner> = ({
 }) => {
   const { tabs, state, dispatch } =
     useModalContext<Array<IModalTabDeclaration<Record<string, any>>>>();
-  const { selectedTabId, defaultSelectedTabId } = state.meta;
+  const { selectedTabId } = state.meta;
   const tabbedModalHTMLId = useGeneratedHtmlId({ prefix: 'tabbedModal' });
-  const tabbedModalHeadingHTMLId = useGeneratedHtmlId({ prefix: 'tabbedModal' });
-  const defaultTabCoordinates = useRef(new Map<string, Pick<DOMRect, 'top'>>());
-  const [translateYValue, setTranslateYValue] = useState(0);
-
-  const onTabContentRender = useCallback(() => {
-    const tabbedModal = document.querySelector(`[id="${tabbedModalHTMLId}"]`) as HTMLDivElement;
-
-    if (!defaultTabCoordinates.current.get(defaultSelectedTabId)) {
-      // on initial render the modal animates into it's final position
-      // hence the need to wait till said animation has completed
-      tabbedModal.onanimationend = () => {
-        const { top } = tabbedModal.getBoundingClientRect();
-        defaultTabCoordinates.current.set(defaultSelectedTabId, { top });
-      };
-    } else {
-      let translateYOverride = 0;
-
-      if (defaultSelectedTabId !== selectedTabId) {
-        const defaultTabData = defaultTabCoordinates.current.get(defaultSelectedTabId);
-
-        const rect = tabbedModal.getBoundingClientRect();
-
-        translateYOverride = translateYValue + (defaultTabData?.top! - rect.top);
-      }
-
-      if (translateYOverride !== translateYValue) {
-        setTranslateYValue(translateYOverride);
-      }
-    }
-  }, [tabbedModalHTMLId, defaultSelectedTabId, selectedTabId, translateYValue]);
+  const tabbedModalHeadingHTMLId = useGeneratedHtmlId({ prefix: 'tabbedModalHeading' });
 
   const selectedTabState = useMemo(
     () => (selectedTabId ? state[selectedTabId] : {}),
@@ -152,12 +121,6 @@ const TabbedModalInner: FC<ITabbedModalInner> = ({
     );
   }, [onSelectedTabChanged, selectedTabId, tabs]);
 
-  const modalPositionOverrideStyles: React.CSSProperties = {
-    transform: `translateY(${translateYValue}px)`,
-    transformOrigin: 'top',
-    willChange: 'transform',
-  };
-
   return (
     <EuiModal
       id={tabbedModalHTMLId}
@@ -169,36 +132,32 @@ const TabbedModalInner: FC<ITabbedModalInner> = ({
       data-test-subj={props['data-test-subj']}
       css={{
         ...(modalWidth ? { width: modalWidth } : {}),
-        ...modalPositionOverrideStyles,
       }}
       aria-labelledby={tabbedModalHeadingHTMLId}
     >
+      <Global
+        styles={{
+          // overrides so modal retains a fixed position from top of viewport, despite having content of varying heights
+          [`.euiOverlayMask:has([id="${tabbedModalHTMLId}"])`]: {
+            alignItems: 'flex-start',
+            paddingBlockStart: '20vh',
+          },
+        }}
+      />
       <EuiModalHeader>
         <EuiModalHeaderTitle id={tabbedModalHeadingHTMLId}>{modalTitle}</EuiModalHeaderTitle>
       </EuiModalHeader>
       <EuiModalBody>
-        <Fragment>
-          <Fragment>{renderTabs()}</Fragment>
-          <EuiSpacer size="m" />
-          {React.createElement(function RenderSelectedTabContent() {
-            useLayoutEffect(() => {
-              requestAnimationFrame(onTabContentRender);
-            }, []);
-            return (
-              <div
-                css={{ display: 'contents' }}
-                data-test-subj={`tabbedModal-${selectedTabId}-content`}
-              >
-                <SelectedTabContent
-                  {...{
-                    state: selectedTabState,
-                    dispatch,
-                  }}
-                />
-              </div>
-            );
-          })}
-        </Fragment>
+        <Fragment>{renderTabs()}</Fragment>
+        <EuiSpacer size="m" />
+        <div css={{ display: 'contents' }} data-test-subj={`tabbedModal-${selectedTabId}-content`}>
+          <SelectedTabContent
+            {...{
+              state: selectedTabState,
+              dispatch,
+            }}
+          />
+        </div>
       </EuiModalBody>
       {modalActionBtn?.id !== undefined && selectedTabState && (
         <EuiModalFooter>