Fix sanitizer call in ToC if html data is array of strings (jupyterla…

…b#17114)

* Fix html sanitizer, if html data is array of strings

* Better use \n, when joining html data

* Add test for cell headings multiline HTML problem

* Harmonize the test description

---------

Co-authored-by: Michał Krassowski <5832902+krassowski@users.noreply.github.com>

packages/cells/src/widget.ts

@@ -1312,9 +1312,13 @@ export class CodeCell extends Cell<ICodeCellModel> {
 
         // Parse HTML output
         if (htmlType) {
+          let htmlData = m.data[htmlType] as string | string[];
+          if (typeof htmlData !== 'string') {
+            htmlData = htmlData.join('\n');
+          }
           headings.push(
             ...TableOfContentsUtils.getHTMLHeadings(
-              this._rendermime.sanitizer.sanitize(m.data[htmlType] as string)
+              this._rendermime.sanitizer.sanitize(htmlData)
             ).map(heading => {
               return {
                 ...heading,

packages/cells/test/widget.spec.ts

@@ -546,6 +546,27 @@ describe('cells/widget', () => {
       });
     });
 
+    describe('#headings', () => {
+      it('should not throw when outputs contain an array of strings', () => {
+        const modelheadings = new CodeCellModel();
+        const widget = new CodeCell({
+          contentFactory: NBTestUtils.createCodeCellFactory(),
+          model: modelheadings,
+          rendermime
+        });
+        widget.initializeState();
+        widget.model.outputs.add({
+          output_type: 'display_data',
+          data: {
+            'text/html': ['Some ', ' thing']
+          },
+          metadata: {}
+        });
+        // try if headings got a problem with multi line html
+        expect(widget.headings).toEqual([]);
+      });
+    });
+
     describe('#outputArea', () => {
       it('should be the output area used by the cell', () => {
         const widget = new CodeCell({