🐛 Fix TypeError when deleting text at edge spans in text editor

The text editor's SelectionController threw 'TypeError: Invalid text
node' when:

- Pressing Backspace to delete the only character of the **first** text
  span in a paragraph that contains multiple spans.
- Pressing Delete to delete the only character of the **last** text
  span in the same situation.
- Pressing a word-backward shortcut that empties the first span of a
  multi-span paragraph.

In all three cases the tree-iterator (previousNode / nextNode) returned
null because no sibling text node existed in that direction, and that
null was subsequently passed to getTextNodeLength() which calls
isTextNode() — which unconditionally throws when given a falsy value.

Fix: use the null-coalescing fallback to the first/last remaining
child's text node of the paragraph before calling collapse() /
getTextNodeLength().

frontend/text-editor/src/editor/controllers/SelectionController.js

@@ -1238,7 +1238,11 @@ export class SelectionController extends EventTarget {
       textSpan.childNodes.length === 0
     ) {
       textSpan.remove();
-      return this.collapse(nextTextNode, 0);
+      // nextTextNode can be null when deleting the last text node in the last
+      // span of the paragraph; fall back to the last text node of the
+      // preceding sibling span so the cursor stays within the paragraph.
+      const forwardTarget = nextTextNode ?? paragraph.lastChild?.lastChild;
+      return this.collapse(forwardTarget, 0);
     }
     return this.collapse(this.focusNode, this.focusOffset);
   }
@@ -1284,9 +1288,14 @@ export class SelectionController extends EventTarget {
       textSpan.childNodes.length === 0
     ) {
       textSpan.remove();
+      // previousTextNode can be null when deleting the first text node in
+      // the paragraph (no preceding sibling text node exists).  Fall back
+      // to the first text node of the now-first remaining span so the
+      // cursor stays within the paragraph.
+      const backwardTarget = previousTextNode ?? paragraph.firstChild?.firstChild;
       return this.collapse(
-        previousTextNode,
-        getTextNodeLength(previousTextNode),
+        backwardTarget,
+        getTextNodeLength(backwardTarget),
       );
     }
 
@@ -1358,9 +1367,13 @@ export class SelectionController extends EventTarget {
         textSpan.childNodes.length === 0
       ) {
         textSpan.remove();
+        // previousTextNode can be null when the deleted node was the first
+        // in the paragraph.  Fall back to the first text node of the
+        // now-first remaining span.
+        const backwardTarget = previousTextNode ?? paragraph.firstChild?.firstChild;
         return this.collapse(
-          previousTextNode,
-          getTextNodeLength(previousTextNode),
+          backwardTarget,
+          getTextNodeLength(backwardTarget),
         );
       }
     }

frontend/text-editor/src/editor/controllers/SelectionController.test.js

@@ -581,6 +581,27 @@ describe("SelectionController", () => {
     expect(textEditorMock.root.textContent).toBe("");
   });
 
+  test("`removeBackwardText` should not throw when deleting the first character of the first span in a multi-span paragraph", () => {
+    // Regression test: previousNode() returns null when the cursor is at the
+    // very first text node; passing null to getTextNodeLength used to throw
+    // "TypeError: Invalid text node".
+    const textEditorMock = TextEditorMock.createTextEditorMockWith([
+      ["A", "B"],
+    ]);
+    const root = textEditorMock.root;
+    const selection = document.getSelection();
+    const selectionController = new SelectionController(
+      textEditorMock,
+      selection,
+    );
+    // Focus at offset 1 of the first span's text node ("A"), then delete backward
+    const firstTextNode = root.firstChild.firstChild.firstChild;
+    focus(selection, textEditorMock, firstTextNode, 1);
+    expect(() => selectionController.removeBackwardText()).not.toThrow();
+    // "A" is removed; the paragraph should keep the second span with "B"
+    expect(textEditorMock.root.textContent).toBe("B");
+  });
+
   test("`insertParagraph` should insert a new paragraph in an empty editor", () => {
     const textEditorMock = TextEditorMock.createTextEditorMockEmpty();
     const root = textEditorMock.root;
@@ -875,6 +896,26 @@ describe("SelectionController", () => {
     );
   });
 
+  test("`removeForwardText` should not throw when deleting the last character of the last span in a multi-span paragraph", () => {
+    // Regression test: nextNode() returns null when the cursor is at the
+    // very last text node; passing null to collapse used to crash.
+    const textEditorMock = TextEditorMock.createTextEditorMockWith([
+      ["A", "B"],
+    ]);
+    const root = textEditorMock.root;
+    const selection = document.getSelection();
+    const selectionController = new SelectionController(
+      textEditorMock,
+      selection,
+    );
+    // Focus at offset 0 of the second span's text node ("B"), then delete forward
+    const secondTextNode = root.firstChild.lastChild.firstChild;
+    focus(selection, textEditorMock, secondTextNode, 0);
+    expect(() => selectionController.removeForwardText()).not.toThrow();
+    // "B" is removed; the paragraph should keep the first span with "A"
+    expect(textEditorMock.root.textContent).toBe("A");
+  });
+
   test("`replaceText` should replace the selected text", () => {
     const textEditorMock =
       TextEditorMock.createTextEditorMockWithText("Hello, World!");