feat(Table): separate sticky and pinned styles

packages/react-table/src/components/Table/Table.tsx

@@ -82,12 +82,14 @@ interface TableContextProps {
   registerSelectableRow?: () => void;
   hasAnimations?: boolean;
   variant?: TableVariant | 'compact';
+  isStickyHeader?: boolean;
 }
 
 export const TableContext = createContext<TableContextProps>({
   registerSelectableRow: () => {},
   hasAnimations: false,
-  variant: undefined
+  variant: undefined,
+  isStickyHeader: false
 });
 
 const TableBase: React.FunctionComponent<TableProps> = ({
@@ -211,7 +213,7 @@ const TableBase: React.FunctionComponent<TableProps> = ({
   };
 
   return (
-    <TableContext.Provider value={{ registerSelectableRow, hasAnimations, variant }}>
+    <TableContext.Provider value={{ registerSelectableRow, hasAnimations, variant, isStickyHeader }}>
       <table
         aria-label={ariaLabel}
         role={role}

packages/react-table/src/components/Table/Thead.tsx

@@ -1,6 +1,33 @@
-import { forwardRef } from 'react';
+import { forwardRef, useCallback, useContext, useEffect, useRef, useState } from 'react';
 import { css } from '@patternfly/react-styles';
 import styles from '@patternfly/react-styles/css/components/Table/table';
+import { TableContext } from './Table';
+
+/** Ratio must be below this to count as “pinned” (avoids doc-layout subpixel + strict threshold: [1] never hitting exactly 1). */
+const PINNED_INTERSECTION_RATIO = 0.999;
+
+const getOverflowScrollParent = (node: HTMLElement): Element | null => {
+  let parent = node.parentElement;
+  while (parent) {
+    const style = getComputedStyle(parent);
+    if (/(auto|scroll|overlay)/.test(style.overflowY) || /(auto|scroll|overlay)/.test(style.overflowX)) {
+      return parent;
+    }
+    parent = parent.parentElement;
+  }
+  return null;
+};
+
+const assignRef = <T,>(ref: React.Ref<T> | undefined, value: T | null) => {
+  if (!ref) {
+    return;
+  }
+  if (typeof ref === 'function') {
+    ref(value);
+  } else {
+    (ref as React.MutableRefObject<T | null>).current = value;
+  }
+};
 
 export interface TheadProps extends React.HTMLProps<HTMLTableSectionElement> {
   /** Content rendered inside the <thead> row group */
@@ -22,20 +49,62 @@ const TheadBase: React.FunctionComponent<TheadProps> = ({
   innerRef,
   hasNestedHeader,
   ...props
-}: TheadProps) => (
-  <thead
-    className={css(
-      styles.tableThead,
-      className,
-      noWrap && styles.modifiers.nowrap,
-      hasNestedHeader && styles.modifiers.nestedColumnHeader
-    )}
-    ref={innerRef}
-    {...props}
-  >
-    {children}
-  </thead>
-);
+}: TheadProps) => {
+  const { isStickyHeader } = useContext(TableContext);
+  const observeStickyPin = !!isStickyHeader;
+  const [isPinned, setIsPinned] = useState(false);
+  const theadElRef = useRef<HTMLTableSectionElement | null>(null);
+
+  const setTheadRef = useCallback(
+    (node: HTMLTableSectionElement | null) => {
+      theadElRef.current = node;
+      assignRef(innerRef, node);
+    },
+    [innerRef]
+  );
+
+  useEffect(() => {
+    if (!observeStickyPin || typeof IntersectionObserver === 'undefined') {
+      setIsPinned(false);
+      return;
+    }
+
+    const el = theadElRef.current;
+    if (!el) {
+      return;
+    }
+
+    const scrollRoot = getOverflowScrollParent(el);
+
+    // Requires sticky thead `inset-block-start: -1px` in CSS (see table.css).
+    const observer = new IntersectionObserver(
+      ([entry]) => {
+        // console.log(scrollRoot, entry, entry.intersectionRatio);
+        setIsPinned(entry.intersectionRatio < PINNED_INTERSECTION_RATIO);
+      },
+      { threshold: [0, 1], root: scrollRoot }
+    );
+
+    observer.observe(el);
+    return () => observer.disconnect();
+  }, [observeStickyPin]);
+
+  return (
+    <thead
+      className={css(
+        styles.tableThead,
+        className,
+        noWrap && styles.modifiers.nowrap,
+        hasNestedHeader && styles.modifiers.nestedColumnHeader,
+        observeStickyPin && isPinned && 'PINNED'
+      )}
+      ref={setTheadRef}
+      {...props}
+    >
+      {children}
+    </thead>
+  );
+};
 
 export const Thead = forwardRef((props: TheadProps, ref: React.Ref<HTMLTableSectionElement>) => (
   <TheadBase {...props} innerRef={ref} />