Treat lists with a single empty item as plain text, not Markdown.

src/Markdown.ts

@@ -112,6 +112,10 @@ const innerNodeLiteral = (node: commonmark.Node): string => {
     return literal;
 };
 
+const emptyItemWithNoSiblings = (node: commonmark.Node): boolean => {
+    return !node.prev && !node.next && !node.firstChild;
+};
+
 /**
  * Class that wraps commonmark, adding the ability to see whether
  * a given message actually uses any markdown syntax or whether
@@ -242,13 +246,30 @@ export default class Markdown {
 
     public isPlainText(): boolean {
         const walker = this.parsed.walker();
-
         let ev: commonmark.NodeWalkingStep | null;
+
         while ((ev = walker.next())) {
             const node = ev.node;
+
             if (TEXT_NODES.indexOf(node.type) > -1) {
                 // definitely text
                 continue;
+            } else if (node.type == "list" || node.type == "item") {
+                // Special handling for inputs like `+`, `*`, `-` and `2021.` which
+                // would otherwise be treated as a list of a single empty item.
+                // See https://github.com/vector-im/element-web/issues/7631
+                if (node.type == "list" && node.firstChild && emptyItemWithNoSiblings(node.firstChild)) {
+                    // A list with a single empty item is treated as plain text.
+                    continue;
+                }
+
+                if (node.type == "item" && emptyItemWithNoSiblings(node)) {
+                    // An empty list item with no sibling items is treated as plain text.
+                    continue;
+                }
+
+                // Everything else is actual lists and therefore not plaintext.
+                return false;
             } else if (node.type == "html_inline" || node.type == "html_block") {
                 // if it's an allowed html tag, we need to render it and therefore
                 // we will need to use HTML. If it's not allowed, it's not HTML since

test/editor/serialize-test.ts

@@ -81,6 +81,29 @@ describe("editor/serialize", function () {
             const html = htmlSerializeIfNeeded(model, {});
             expect(html).toBe("*hello* world < hey world!");
         });
+
+        it("lists with a single empty item are not considered markdown", function () {
+            const pc = createPartCreator();
+
+            const model1 = new EditorModel([pc.plain("-")], pc);
+            const html1 = htmlSerializeIfNeeded(model1, {});
+            expect(html1).toBe(undefined);
+
+            const model2 = new EditorModel([pc.plain("* ")], pc);
+            const html2 = htmlSerializeIfNeeded(model2, {});
+            expect(html2).toBe(undefined);
+
+            const model3 = new EditorModel([pc.plain("2021.")], pc);
+            const html3 = htmlSerializeIfNeeded(model3, {});
+            expect(html3).toBe(undefined);
+        });
+
+        it("lists with a single non-empty item are still markdown", function () {
+            const pc = createPartCreator();
+            const model = new EditorModel([pc.plain("2021. foo")], pc);
+            const html = htmlSerializeIfNeeded(model, {});
+            expect(html).toBe('<ol start="2021">\n<li>foo</li>\n</ol>\n');
+        });
     });
 
     describe("with plaintext", function () {