tools: fix CJS/ESM toggle on small screens

PR-URL: https://github.com/nodejs/node/pull/43506
Co-authored-by: Moshe Atlow <moshe@atlow.co.il>
Fixes: https://github.com/nodejs/node/issues/43468
Reviewed-By: LiviaMedeiros <livia@cirno.name>

3 files changed, 74 insertions, 2 deletions

diff --git a/tools/doc/allhtml.mjs b/tools/doc/allhtml.mjs
index 50f145034cc..fb0b719ac8a 100644
--- a/tools/doc/allhtml.mjs
+++ b/tools/doc/allhtml.mjs
@@ -2,6 +2,7 @@
 // of the generated html files.
 
 import fs from 'fs';
+import buildCSSForFlavoredJS from './buildCSSForFlavoredJS.mjs';
 
 const source = new URL('../../out/doc/api/', import.meta.url);
 
@@ -90,7 +91,14 @@ all = all.slice(0, tocStart.index + tocStart[0].length) +
 // Replace apicontent with the concatenated set of apicontents from each source.
 const apiStart = /<\w+ id="apicontent">\s*/.exec(all);
 const apiEnd = all.lastIndexOf('<!-- API END -->');
-all = all.slice(0, apiStart.index + apiStart[0].length) +
+all = all.slice(0, apiStart.index + apiStart[0].length)
+    .replace(
+      '\n</head>',
+      buildCSSForFlavoredJS(new Set(Array.from(
+        apicontent.matchAll(/(?<=<pre class="with-)\d+(?=-chars">)/g),
+        (x) => Number(x[0])
+      ))) + '\n</head>'
+    ) +
   apicontent +
   all.slice(apiEnd);
 
diff --git a/tools/doc/buildCSSForFlavoredJS.mjs b/tools/doc/buildCSSForFlavoredJS.mjs
new file mode 100644
index 00000000000..6b355ab1760
--- /dev/null
+++ b/tools/doc/buildCSSForFlavoredJS.mjs
@@ -0,0 +1,53 @@
+const CHAR_THRESHOLD = 68; // Around 68 characters, we have to take into
+//                            account the left column that appears on screen
+//                            wider than 1024px.
+
+const ESTIMATED_CHAR_WIDTH = 8; // If the root element font-size is 16px (default value), 1ch is 7.8025px.
+const TOGGLE_WIDTH = 142; // https://github.com/nodejs/node/blob/dbd938549b16718f2e4cc2809746b8c44a191b1d/doc/api_assets/style.css#L954
+
+const PRE_MARGIN_LEFT = 16; // https://github.com/nodejs/node/blob/dbd938549b16718f2e4cc2809746b8c44a191b1d/doc/api_assets/style.css#L516
+const PRE_MARGIN_RIGHT = 16; // https://github.com/nodejs/node/blob/dbd938549b16718f2e4cc2809746b8c44a191b1d/doc/api_assets/style.css#L516
+const PRE_PADDING_LEFT = 16; // https://github.com/nodejs/node/blob/dbd938549b16718f2e4cc2809746b8c44a191b1d/doc/api_assets/style.css#L513
+const PRE_PADDING_RIGHT = 16; // https://github.com/nodejs/node/blob/dbd938549b16718f2e4cc2809746b8c44a191b1d/doc/api_assets/style.css#L513
+
+
+const COLUMN_RIGHT_MARGIN_LEFT_LARGE_SCREEN = 234; // https://github.com/nodejs/node/blob/dbd938549b16718f2e4cc2809746b8c44a191b1d/doc/api_assets/style.css#L653
+const COLUMN_RIGHT_MARGIN_LEFT_SMALL_SCREEN = 0; // https://github.com/nodejs/node/blob/dbd938549b16718f2e4cc2809746b8c44a191b1d/doc/api_assets/style.css#L906
+const COLUMN_RIGHT_MARGIN_RIGHT_LARGE_SCREEN = 0;
+const COLUMN_RIGHT_MARGIN_RIGHT_SMALL_SCREEN = 0;
+const COLUMN_RIGHT_PADDING_LEFT_LARGE_SCREEN = 24; // https://github.com/nodejs/node/blob/dbd938549b16718f2e4cc2809746b8c44a191b1d/doc/api_assets/style.css#L655
+const COLUMN_RIGHT_PADDING_LEFT_SMALL_SCREEN = 8; // https://github.com/nodejs/node/blob/dbd938549b16718f2e4cc2809746b8c44a191b1d/doc/api_assets/style.css#L907
+const COLUMN_RIGHT_PADDING_RIGHT_LARGE_SCREEN = 32; // https://github.com/nodejs/node/blob/dbd938549b16718f2e4cc2809746b8c44a191b1d/doc/api_assets/style.css#L654
+const COLUMN_RIGHT_PADDING_RIGHT_SMALL_SCREEN = 8; // https://github.com/nodejs/node/blob/dbd938549b16718f2e4cc2809746b8c44a191b1d/doc/api_assets/style.css#L908
+
+const getMarginLeft = (charCount) =>
+  (charCount > CHAR_THRESHOLD ?
+    COLUMN_RIGHT_MARGIN_LEFT_LARGE_SCREEN :
+    COLUMN_RIGHT_MARGIN_LEFT_SMALL_SCREEN) + PRE_MARGIN_LEFT;
+const getPaddingLeft = (charCount) =>
+  (charCount > CHAR_THRESHOLD ?
+    COLUMN_RIGHT_PADDING_LEFT_LARGE_SCREEN :
+    COLUMN_RIGHT_PADDING_LEFT_SMALL_SCREEN) + PRE_PADDING_LEFT;
+const getPaddingRight = (charCount) =>
+  (charCount > CHAR_THRESHOLD ?
+    COLUMN_RIGHT_PADDING_RIGHT_LARGE_SCREEN :
+    COLUMN_RIGHT_PADDING_RIGHT_SMALL_SCREEN) + PRE_PADDING_RIGHT;
+const getMarginRight = (charCount) =>
+  (charCount > CHAR_THRESHOLD ?
+    COLUMN_RIGHT_MARGIN_RIGHT_LARGE_SCREEN :
+    COLUMN_RIGHT_MARGIN_RIGHT_SMALL_SCREEN) + PRE_MARGIN_RIGHT;
+
+
+export default function buildCSSForFlavoredJS(dynamicSizes) {
+  if (dynamicSizes == null || dynamicSizes.length === 0) return '';
+
+  return `<style>${Array.from(dynamicSizes, (charCount) =>
+    `@media(max-width:${getMarginLeft(charCount) + getPaddingLeft(charCount) +
+                        charCount * ESTIMATED_CHAR_WIDTH + TOGGLE_WIDTH +
+                        getPaddingRight(charCount) + getMarginRight(charCount)}px){` +
+      `.with-${charCount}-chars>.js-flavor-selector{` +
+        'float:none;' +
+        'margin:0 0 1em auto;' +
+      '}' +
+    '}').join('')}</style>`;
+}
diff --git a/tools/doc/html.mjs b/tools/doc/html.mjs
index 1caffa7158c..373f5487a31 100644
--- a/tools/doc/html.mjs
+++ b/tools/doc/html.mjs
@@ -33,6 +33,9 @@ import { visit } from 'unist-util-visit';
 
 import * as common from './common.mjs';
 import * as typeParser from './type-parser.mjs';
+import buildCSSForFlavoredJS from './buildCSSForFlavoredJS.mjs';
+
+const dynamicSizes = Object.create(null);
 
 const { highlight, getLanguage } = highlightJs;
 
@@ -90,6 +93,8 @@ function processContent(content) {
 }
 
 export function toHTML({ input, content, filename, nodeVersion, versions }) {
+  const dynamicSizesForThisFile = dynamicSizes[filename];
+
   filename = path.basename(filename, '.md');
 
   const id = filename.replace(/\W+/g, '-');
@@ -99,6 +104,7 @@ export function toHTML({ input, content, filename, nodeVersion, versions }) {
                      .replace('__SECTION__', content.section)
                      .replace(/__VERSION__/g, nodeVersion)
                      .replace(/__TOC__/g, content.toc)
+                     .replace('__JS_FLAVORED_DYNAMIC_CSS__', buildCSSForFlavoredJS(dynamicSizesForThisFile))
                      .replace(/__TOC_PICKER__/g, tocPicker(id, content))
                      .replace(/__GTOC_PICKER__/g, gtocPicker(id))
                      .replace(/__GTOC__/g, gtocHTML.replace(
@@ -228,14 +234,19 @@ export function preprocessElements({ filename }) {
           const previousNode = parent.children[index - 1] || {};
           const nextNode = parent.children[index + 1] || {};
 
+          const charCountFirstTwoLines = Math.max(...node.value.split('\n', 2).map((str) => str.length));
+
           if (!isJSFlavorSnippet(previousNode) &&
               isJSFlavorSnippet(nextNode) &&
               nextNode.lang !== node.lang) {
             // Saving the highlight code as value to be added in the next node.
             node.value = highlighted;
+            node.charCountFirstTwoLines = charCountFirstTwoLines;
           } else if (isJSFlavorSnippet(previousNode) &&
                      previousNode.lang !== node.lang) {
-            node.value = '<pre>' +
+            const actualCharCount = Math.max(charCountFirstTwoLines, previousNode.charCountFirstTwoLines);
+            (dynamicSizes[filename] ??= new Set()).add(actualCharCount);
+            node.value = `<pre class="with-${actualCharCount}-chars">` +
               '<input class="js-flavor-selector" type="checkbox"' +
               // If CJS comes in second, ESM should display by default.
               (node.lang === 'cjs' ? ' checked' : '') +