Fix ToastUI editor bug preventing manual codeblock insertion

Manually inserting a codeblock adding three backticks and hitting enter
is not functioning in the ToastUI editor due to an existing bug in the library.
This commit implements a workaround to address the issue.

app/assets/javascripts/markdown_editor.js

@@ -6,6 +6,39 @@
  *
  */
 
+const BACKTICK = 192;
+// These counters can be global. Scoping them to each editor becomes redundant
+// since they are reset to this state when switching editors. 
+// Read more: https://github.com/openHPI/codeocean/pull/2242#discussion_r1576617432
+let backtickPressedCount = 0;
+let justInsertedCodeBlock = false;
+
+const deleteSelection = (editor, count) => {
+  // The backtick is a so-called dead key, which is waiting for further input to be combined with.
+  // For example a backtick and the letter a are combined to à.
+  // When we remove a selection ending with a backtick, we want to clear the keyboard buffer, too.
+  // This ensures that typing a regular character a after this operation is not combined into à, but just inserted as a.
+  // This solution is taken from https://stackoverflow.com/a/72634132.
+  editor.blur();
+  setTimeout(() => editor.focus());
+  // Get current position
+  const selectionRange = editor.getSelection();
+  // Replace the previous `count` characters with an empty string.
+  // We use a replace function (rather than delete) to avoid issues with line breaks in ToastUi.
+  // Otherwise, a line break following the cursor position might still be displayed normally,
+  // but could be removed erroneously from the internal editor state.
+  // If this happens, code blocks ending with \n``` are not recognized correctly.
+  editor.replaceSelection(
+    "",
+    [selectionRange[0][0], selectionRange[0][1] - count],
+    [selectionRange[1][0], selectionRange[1][1]]
+  );
+};
+const resetCount = (withBlock = false) => {
+  backtickPressedCount = 0;
+  justInsertedCodeBlock = withBlock;
+};
+
 const initializeMarkdownEditors = () => {
   const editors = document.querySelectorAll(
     '[data-behavior="markdown-editor-widget"]'
@@ -42,6 +75,39 @@ const initializeMarkdownEditors = () => {
           const content = toastEditor.getMarkdown();
           formInput.value = content;
         },
+        // Fix ToastUI editor bug preventing manual codeblock insertion:
+        // Manually inserting a codeblock adding three backticks and hitting enter
+        // is not functioning in the ToastUI editor due to an existing bug in the library.
+        // Therefore, this `keyup` handler implements a workaround to address the issue.
+        keyup: (_, event) => {
+          // Although the use of keyCode seems to be deprecated, the suggested alternatives (key or code)
+          // work inconsistently across browsers. Using keyCode works flawless for now.
+          // Read more: https://github.com/openHPI/codeocean/pull/2242#discussion_r1576675620
+          if (event.keyCode === BACKTICK) {
+            backtickPressedCount++;
+            if (backtickPressedCount === 2) {
+              // Remove the last two backticks and insert a code block
+              // The order of operations is important here: Inserting the code block first and then removing
+              // some backticks won't work, since this would infer with the internal ToastUi editor state.
+              // With the current solution, we don't mingle with the code block inserted by ToastUi at all.
+              deleteSelection(toastEditor, 2);
+              toastEditor.exec("codeBlock");
+              resetCount(true);
+            }
+          } else if (backtickPressedCount === 1 && justInsertedCodeBlock) {
+            // We want to improve the usage of our code block fix with the following mechanism.
+            // Usually, three backticks are required to start a code block.
+            // However, with our workaround only two backticks are required.
+            // Out of habit, however, users might still enter three backticks at once,
+            // not noticing that the code block was already inserted after the second one.
+            // Thus, we remove one additional backtick entered after starting a code block through our fix.
+            deleteSelection(toastEditor, 1);
+            resetCount();
+          } else {
+            // If any other key is pressed, reset the count
+            resetCount();
+          }
+        },
       },
     });