turndown: Paste single line of code as inline code in markdown.

When pasting any code without any newlines, whether from a code block or
a code span, it will always be pasted as inline code.

That is, a line of code copied from within a code span will retain code
span formatting on paste, and a line of code copied from within a code
block will be pasted as inline code, instead of a 1 line code block.

web/src/copy_and_paste.js

@@ -319,7 +319,7 @@ export function paste_handler_converter(paste_html) {
         copied_html_fragment.childNodes.length === 1 &&
         copied_html_fragment.firstElementChild &&
         copied_html_fragment.firstElementChild.innerHTML;
-    const outer_elements_to_retain = ["PRE", "UL", "OL", "A"];
+    const outer_elements_to_retain = ["PRE", "UL", "OL", "A", "CODE"];
     // If the entire selection copied is within a single HTML element (like an
     // `h1`), we don't want to retain its styling, except when it is needed to
     // identify the intended structure of the copied content.
@@ -481,6 +481,62 @@ export function paste_handler_converter(paste_html) {
         },
     });
 
+    // We override the original upstream implementation of this rule to turn any
+    // single line code blocks into inline markdown code. Everything else is the same.
+    turndownService.addRule("fencedCodeBlock", {
+        filter(node, options) {
+            return (
+                options.codeBlockStyle === "fenced" &&
+                node.nodeName === "PRE" &&
+                node.firstChild &&
+                node.firstChild.nodeName === "CODE"
+            );
+        },
+
+        replacement(_content, node, options) {
+            const code = node.firstChild.textContent;
+
+            // We convert single line code inside a code block to inline markdown code,
+            // and the code for this is taken from upstream's `code` rule.
+            if (!code.includes("\n")) {
+                if (!code) {
+                    return "";
+                }
+                const extraSpace = /^`|^ .*?[^ ].* $|`$/.test(code) ? " " : "";
+
+                // Pick the shortest sequence of backticks that is not found in the code
+                // to be the delimiter.
+                let delimiter = "`";
+                const matches = code.match(/`+/gm) || [];
+                while (matches.includes(delimiter)) {
+                    delimiter = delimiter + "`";
+                }
+
+                return delimiter + extraSpace + code + extraSpace + delimiter;
+            }
+
+            const className = node.firstChild.getAttribute("class") || "";
+            const language = (className.match(/language-(\S+)/) || [null, ""])[1];
+
+            const fenceChar = options.fence.charAt(0);
+            let fenceSize = 3;
+            const fenceInCodeRegex = new RegExp("^" + fenceChar + "{3,}", "gm");
+
+            let match;
+            while ((match = fenceInCodeRegex.exec(code))) {
+                if (match[0].length >= fenceSize) {
+                    fenceSize = match[0].length + 1;
+                }
+            }
+
+            const fence = fenceChar.repeat(fenceSize);
+
+            return (
+                "\n\n" + fence + language + "\n" + code.replace(/\n$/, "") + "\n" + fence + "\n\n"
+            );
+        },
+    });
+
     let markdown_text = turndownService.turndown(paste_html);
 
     // Checks for escaped ordered list syntax.

web/tests/copy_and_paste.test.js

@@ -35,6 +35,11 @@ run_test("paste_handler_converter", () => {
         'zulip code block in python\n\n```Python\nprint("hello world")\n```',
     );
 
+    // Single line in a code block
+    input =
+        '<meta http-equiv="content-type" content="text/html; charset=utf-8"><pre><code>single line</code></pre>';
+    assert.equal(copy_and_paste.paste_handler_converter(input), "`single line`");
+
     // Raw links without custom text
     input =
         '<meta http-equiv="content-type" content="text/html; charset=utf-8"><a href="https://zulip.readthedocs.io/en/latest/subsystems/logging.html" target="_blank" title="https://zulip.readthedocs.io/en/latest/subsystems/logging.html" style="color: hsl(200, 100%, 40%); text-decoration: none; cursor: pointer; font-family: &quot;Source Sans 3&quot;, &quot;Helvetica Neue&quot;, Helvetica, Arial, sans-serif; font-size: 14px; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; background-color: hsl(0, 0%, 100%);">https://zulip.readthedocs.io/en/latest/subsystems/logging.html</a>';