fix mermaid charts in dark mode (#11754)

* fix mermaid charts in dark mode

* fix linting error

Author: a-hariti

src/components/mermaid.tsx

@@ -1,9 +1,16 @@
 'use client';
-import {useEffect} from 'react';
+import {useEffect, useState} from 'react';
 import {useTheme} from 'next-themes';
 
+/**
+ * we target ```mermaid``` code blocks after they have been highlighted (not ideal),
+ * then we strip the code from the html elements used for highlighting
+ * then we render the mermaid chart both in light and dark modes
+ * CSS takes care of showing the right one depending on the theme
+ */
 export default function Mermaid() {
-  const theme = useTheme();
+  const [isDoneRendering, setDoneRendering] = useState(false);
+  const {resolvedTheme: theme} = useTheme();
   useEffect(() => {
     const renderMermaid = async () => {
       const escapeHTML = (str: string) => {
@@ -24,24 +31,64 @@ export default function Mermaid() {
         return;
       }
       const {default: mermaid} = await import('mermaid');
-      mermaid.initialize({
-        startOnLoad: false,
-        theme: theme.resolvedTheme === 'light' ? 'default' : 'dark',
-      });
-      mermaidBlocks.forEach(block => {
+      mermaid.initialize({startOnLoad: false});
+      mermaidBlocks.forEach(lightModeblock => {
         // get rid of code highlighting
-        const code = block.textContent ?? '';
-        block.innerHTML = escapeHTML(code);
+        const code = lightModeblock.textContent ?? '';
+        lightModeblock.innerHTML = escapeHTML(code);
         // force transparent background
-        block.style.backgroundColor = 'transparent';
-        const parentCodeTabs = block.closest('.code-tabs-wrapper');
-        if (parentCodeTabs) {
-          parentCodeTabs.innerHTML = block.outerHTML;
+        lightModeblock.style.backgroundColor = 'transparent';
+        lightModeblock.classList.add('light');
+        const parentCodeTabs = lightModeblock.closest('.code-tabs-wrapper');
+        if (!parentCodeTabs) {
+          // eslint-disable-next-line no-console
+          console.error('Mermaid code block was not wrapped in a code tab');
+          return;
         }
+        // empty the container
+        parentCodeTabs.innerHTML = '';
+        parentCodeTabs.appendChild(lightModeblock.cloneNode(true));
+
+        const darkModeBlock = lightModeblock.cloneNode(true) as HTMLPreElement;
+        darkModeBlock.classList.add('dark');
+        darkModeBlock.classList.remove('light');
+        parentCodeTabs?.appendChild(darkModeBlock);
       });
-      await mermaid.run({nodes: document.querySelectorAll('.language-mermaid')});
+      await mermaid.run({nodes: document.querySelectorAll('.language-mermaid.light')});
+
+      mermaid.initialize({startOnLoad: false, theme: 'dark'});
+      await mermaid
+        .run({nodes: document.querySelectorAll('.language-mermaid.dark')})
+        .then(() => setDoneRendering(true));
     };
     renderMermaid();
-  }, [theme]);
-  return null;
+  }, []);
+  // we have to wait for mermaid.js to finish rendering both light and dark charts
+  // before we hide one of them depending on the theme
+  // this is necessary because mermaid.js relies on the DOM for calculations
+  return isDoneRendering ? (
+    theme === 'dark' ? (
+      <style>
+        {`
+        .dark .language-mermaid {
+          display: none;
+        }
+        .dark .language-mermaid.dark {
+          display: block;
+        }
+      `}
+      </style>
+    ) : (
+      <style>
+        {`
+        .language-mermaid.light {
+          display: block;
+        }
+        .language-mermaid.dark {
+          display: none;
+        }
+      `}
+      </style>
+    )
+  ) : null;
 }