Add stickyTop prop to PageLayout.Pane

Author: broccolinisoup

docs/content/PageLayout.mdx

@@ -168,6 +168,36 @@ See [storybook](https://primer.style/react/storybook?path=/story/layout-pagelayo
 </Box>
 ```
 
+### With a stickyTop sticky pane
+
+```jsx live
+<Box sx={{height: '360px', overflowY: 'scroll', border: '1px solid', borderColor: 'border.default'}}>
+  <Box
+    height="100px"
+    width="100%"
+    borderBottom="1px solid"
+    borderColor="border.default"
+    sx={{position: 'sticky', top: '0', backgroundColor: 'canvas.subtle'}}
+    display="flex"
+    alignItems="center"
+    justifyContent="center"
+  >
+    <Heading>Sticky top content</Heading>
+  </Box>
+  <PageLayout>
+    <PageLayout.Content>
+      <Placeholder label="Content" height={240} />
+    </PageLayout.Content>
+    <PageLayout.Pane stickyTop={100} position="start" sticky>
+      <Placeholder label="Pane" height={240} />
+    </PageLayout.Pane>
+    <PageLayout.Footer>
+      <Placeholder label="Footer" height={64} />
+    </PageLayout.Footer>
+  </PageLayout>
+</Box>
+```
+
 ## Props
 
 ### PageLayout

src/PageLayout/PageLayout.stories.tsx

@@ -659,4 +659,72 @@ NestedScrollContainer.argTypes = {
   }
 }
 
+export const StickyPaneWithStickyTop: Story = args => (
+  // a box to create a sticky top element that will be on the consumer side and outside of the PageLayout component
+  <Box>
+    <Box
+      height={args.stickyTop}
+      width="100%"
+      border="1px solid"
+      borderColor="border.default"
+      sx={{position: 'sticky', top: '0', backgroundColor: 'canvas.subtle'}}
+      display="flex"
+      alignItems="center"
+      justifyContent="center"
+    >
+      <Heading>Sticky top content</Heading>
+    </Box>
+    <PageLayout rowGap="none" columnGap="none" padding="none" containerWidth="full">
+      <PageLayout.Content padding="normal" width="large">
+        <Box sx={{display: 'grid', gap: 3}}>
+          {Array.from({length: args.numParagraphsInContent}).map((_, i) => (
+            <Box key={i} as="p" sx={{margin: 0}}>
+              Lorem ipsum dolor sit amet, consectetur adipiscing elit. Nam at enim id lorem tempus egestas a non ipsum.
+              Maecenas imperdiet ante quam, at varius lorem molestie vel. Sed at eros consequat, varius tellus et,
+              auctor felis. Donec pulvinar lacinia urna nec commodo. Phasellus at imperdiet risus. Donec sit amet massa
+              purus. Nunc sem lectus, bibendum a sapien nec, tristique tempus felis. Ut porttitor auctor tellus in
+              imperdiet. Ut blandit tincidunt augue, quis fringilla nunc tincidunt sed. Vestibulum auctor euismod nisi.
+              Nullam tincidunt est in mi tincidunt dictum. Sed consectetur aliquet velit ut ornare.
+            </Box>
+          ))}
+        </Box>
+      </PageLayout.Content>
+      <PageLayout.Pane position="start" padding="normal" divider="line" stickyTop={100} sticky>
+        <Box sx={{display: 'grid', gap: 3}}>
+          {Array.from({length: args.numParagraphsInPane}).map((_, i) => (
+            <Box key={i} as="p" sx={{margin: 0}}>
+              Lorem ipsum dolor sit amet, consectetur adipiscing elit. Nam at enim id lorem tempus egestas a non ipsum.
+              Maecenas imperdiet ante quam, at varius lorem molestie vel. Sed at eros consequat, varius tellus et,
+              auctor felis. Donec pulvinar lacinia urna nec commodo. Phasellus at imperdiet risus. Donec sit amet massa
+              purus.
+            </Box>
+          ))}
+        </Box>
+      </PageLayout.Pane>
+      <PageLayout.Footer padding="normal" divider="line">
+        <Placeholder label="Footer" height={64} />
+      </PageLayout.Footer>
+    </PageLayout>
+  </Box>
+)
+
+StickyPaneWithStickyTop.argTypes = {
+  sticky: {
+    type: 'boolean',
+    defaultValue: true
+  },
+  stickyTop: {
+    type: 'number',
+    defaultValue: 100
+  },
+  numParagraphsInPane: {
+    type: 'number',
+    defaultValue: 10
+  },
+  numParagraphsInContent: {
+    type: 'number',
+    defaultValue: 30
+  }
+}
+
 export default meta

src/PageLayout/PageLayout.tsx

@@ -22,7 +22,7 @@ const PageLayoutContext = React.createContext<{
   padding: keyof typeof SPACING_MAP
   rowGap: keyof typeof SPACING_MAP
   columnGap: keyof typeof SPACING_MAP
-  enableStickyPane?: () => void
+  enableStickyPane?: (stickyTopHeight: number) => void
   disableStickyPane?: () => void
   contentTopRef?: (node?: Element | null | undefined) => void
   contentBottomRef?: (node?: Element | null | undefined) => void
@@ -362,6 +362,7 @@ export type PageLayoutPaneProps = {
    */
   dividerWhenNarrow?: 'inherit' | 'none' | 'line' | 'filled'
   sticky?: boolean
+  stickyTop?: number // the height of the sticky top element
   hidden?: boolean | ResponsiveValue<boolean>
 } & SxProp
 
@@ -384,6 +385,7 @@ const Pane: React.FC<React.PropsWithChildren<PageLayoutPaneProps>> = ({
   divider: responsiveDivider = 'none',
   dividerWhenNarrow = 'inherit',
   sticky = false,
+  stickyTop = 0,
   hidden: responsiveHidden = false,
   children,
   sx = {}
@@ -410,11 +412,11 @@ const Pane: React.FC<React.PropsWithChildren<PageLayoutPaneProps>> = ({
 
   React.useEffect(() => {
     if (sticky) {
-      enableStickyPane?.()
+      enableStickyPane?.(stickyTop)
     } else {
       disableStickyPane?.()
     }
-  }, [sticky, enableStickyPane, disableStickyPane])
+  }, [sticky, enableStickyPane, disableStickyPane, stickyTop])
 
   return (
     <Box
@@ -439,7 +441,9 @@ const Pane: React.FC<React.PropsWithChildren<PageLayoutPaneProps>> = ({
               ...(sticky
                 ? {
                     position: 'sticky',
-                    top: 0,
+                    // If stickyTop has value, it will stick the pane to the position where the sticky top ends
+                    // else top will be 0 as the default value of stickyTop
+                    top: stickyTop,
                     overflow: 'hidden',
                     maxHeight: 'var(--sticky-pane-height)'
                   }

src/PageLayout/useStickyPaneHeight.ts

@@ -11,6 +11,8 @@ export function useStickyPaneHeight() {
 
   // Default the height to the viewport height
   const [height, setHeight] = React.useState('100vh')
+  // stickyTop state
+  const [stickyTopHeight, setStickyTopHeight] = React.useState(0)
 
   // Create intersection observers to track the top and bottom of the content region
   const [contentTopRef, contentTopInView, contentTopEntry] = useInView()
@@ -44,11 +46,11 @@ export function useStickyPaneHeight() {
       // We need to account for this when calculating the offset.
       const overflowScroll = Math.max(window.scrollY + window.innerHeight - document.body.scrollHeight, 0)
 
-      calculatedHeight = `calc(100vh - ${topOffset + bottomOffset - overflowScroll}px)`
+      calculatedHeight = `calc(100vh - ${Math.max(topOffset, stickyTopHeight) + bottomOffset - overflowScroll}px)`
     }
 
     setHeight(calculatedHeight)
-  }, [contentTopEntry, contentBottomEntry])
+  }, [contentTopEntry, contentBottomEntry, stickyTopHeight])
 
   // We only want to add scroll and resize listeners if the pane is sticky.
   // Since hooks can't be called conditionally, we need to use state to track
@@ -88,10 +90,17 @@ export function useStickyPaneHeight() {
     }
   }, [isEnabled, contentTopInView, contentBottomInView, calculateHeight])
 
+  const enableStickyPane = (stickyTop: number) => {
+    setIsEnabled(true)
+    setStickyTopHeight(stickyTop)
+  }
+
+  const disableStickyPane = () => setIsEnabled(false)
+
   return {
     rootRef,
-    enableStickyPane: () => setIsEnabled(true),
-    disableStickyPane: () => setIsEnabled(false),
+    enableStickyPane,
+    disableStickyPane,
     contentTopRef,
     contentBottomRef,
     stickyPaneHeight: height