fix Firefox arrow navigation bug

.gitignore

@@ -79,3 +79,6 @@ lib/
 
 # react-native-live-markdown
 .build_complete
+
+# jest
+coverage/

src/MarkdownTextInput.web.tsx

@@ -24,6 +24,8 @@ import {getElementHeight, getPlaceholderValue, isEventComposing, normalizeValue,
 import {idGenerator, parseToReactDOMStyle, processMarkdownStyle} from './web/utils/webStyleUtils';
 import {forceRefreshAllImages} from './web/inputElements/inlineImage';
 import type {MarkdownRange, InlineImagesInputProps} from './commonTypes';
+import BrowserUtils from './web/utils/browserUtils';
+import {handleFirefoxArrowKeyNavigation} from './web/utils/firefoxUtils';
 
 const useClientEffect = typeof window === 'undefined' ? useEffect : useLayoutEffect;
 
@@ -526,6 +528,15 @@ const MarkdownTextInput = React.forwardRef<MarkdownTextInput, MarkdownTextInputP
           onKeyPress(event);
         }
 
+        // Handle Arrow keys for consistent navigation across grapheme clusters (like emojis) on Firefox
+        if (BrowserUtils.isFirefox && ['ArrowRight', 'ArrowLeft'].includes(e.key) && !nativeEvent.altKey) {
+          e.preventDefault();
+          if (!divRef.current) {
+            return;
+          }
+          handleFirefoxArrowKeyNavigation(divRef.current, nativeEvent?.shiftKey, e.key === 'ArrowRight' ? 'right' : 'left');
+        }
+
         if (
           e.key === 'Enter' &&
           // Do not call submit if composition is occuring.

src/web/utils/__tests__/firefoxUtils.test.ts

@@ -0,0 +1,92 @@
+import * as CursorUtils from '../cursorUtils';
+import type {MarkdownTextInputElement} from '../../../MarkdownTextInput.web';
+import {handleFirefoxArrowKeyNavigation} from '../firefoxUtils';
+
+const createMockTarget = (value: string): MarkdownTextInputElement => {
+  const div = document.createElement('div') as unknown as MarkdownTextInputElement;
+  div.value = value;
+  return div;
+};
+
+jest.mock('../cursorUtils', () => ({
+  ...jest.requireActual('../cursorUtils'),
+  getCurrentCursorPosition: jest.fn(),
+  setCursorPosition: jest.fn(),
+}));
+
+describe('handleFirefoxArrowKeyNavigation', () => {
+  beforeEach(() => {
+    jest.clearAllMocks();
+  });
+
+  it('should do nothing if no cursor in target', () => {
+    const target = createMockTarget('test');
+    (CursorUtils.getCurrentCursorPosition as jest.Mock).mockReturnValue(null);
+
+    handleFirefoxArrowKeyNavigation(target);
+    expect(CursorUtils.setCursorPosition).not.toHaveBeenCalled();
+  });
+
+  it('should move cursor to next grapheme boundary with regular text', () => {
+    const target = createMockTarget('hello world');
+    (CursorUtils.getCurrentCursorPosition as jest.Mock).mockReturnValue({start: 5, end: 5});
+
+    handleFirefoxArrowKeyNavigation(target);
+    expect(CursorUtils.setCursorPosition).toHaveBeenCalledWith(target, 6, 6);
+  });
+
+  it('should move cursor correctly when inside emoji', () => {
+    const target = createMockTarget('😀text');
+    (CursorUtils.getCurrentCursorPosition as jest.Mock).mockReturnValue({start: 1, end: 1});
+
+    handleFirefoxArrowKeyNavigation(target);
+    expect(CursorUtils.setCursorPosition).toHaveBeenCalledWith(target, 2, 2);
+  });
+
+  it('should not move cursor beyond text length', () => {
+    const target = createMockTarget('test');
+
+    (CursorUtils.getCurrentCursorPosition as jest.Mock).mockReturnValue({start: 4, end: 4});
+    handleFirefoxArrowKeyNavigation(target);
+    expect(CursorUtils.setCursorPosition).toHaveBeenCalledWith(target, 4, 4);
+  });
+
+  it('should handle multiple emojis', () => {
+    const target = createMockTarget('😀😀text');
+    (CursorUtils.getCurrentCursorPosition as jest.Mock).mockReturnValueOnce({start: 0, end: 0}).mockReturnValueOnce({start: 2, end: 2});
+
+    handleFirefoxArrowKeyNavigation(target);
+    expect(CursorUtils.setCursorPosition).toHaveBeenCalledWith(target, 2, 2);
+    handleFirefoxArrowKeyNavigation(target);
+    expect(CursorUtils.setCursorPosition).toHaveBeenCalledWith(target, 4, 4);
+  });
+
+  it('should handle multiple emojis backward navigation', () => {
+    const target = createMockTarget('😀😀text');
+    (CursorUtils.getCurrentCursorPosition as jest.Mock).mockReturnValueOnce({start: 4, end: 4}).mockReturnValueOnce({start: 2, end: 2});
+
+    handleFirefoxArrowKeyNavigation(target, false, 'left');
+    expect(CursorUtils.setCursorPosition).toHaveBeenCalledWith(target, 2, 2);
+    handleFirefoxArrowKeyNavigation(target, false, 'left');
+    expect(CursorUtils.setCursorPosition).toHaveBeenCalledWith(target, 2, 2);
+  });
+
+  it('should handle emoji selection', () => {
+    const target = createMockTarget('😀😀text');
+    (CursorUtils.getCurrentCursorPosition as jest.Mock).mockReturnValueOnce({start: 0, end: 0}).mockReturnValueOnce({start: 2, end: 2});
+
+    handleFirefoxArrowKeyNavigation(target, true);
+    expect(CursorUtils.setCursorPosition).toHaveBeenCalledWith(target, 0, 2);
+    handleFirefoxArrowKeyNavigation(target);
+    expect(CursorUtils.setCursorPosition).toHaveBeenCalledWith(target, 4, 4);
+  });
+  it('should handle emoji selection backwards', () => {
+    const target = createMockTarget('😀😀text');
+    (CursorUtils.getCurrentCursorPosition as jest.Mock).mockReturnValueOnce({start: 4, end: 4}).mockReturnValueOnce({start: 2, end: 4});
+
+    handleFirefoxArrowKeyNavigation(target, true, 'left');
+    expect(CursorUtils.setCursorPosition).toHaveBeenCalledWith(target, 2, 4);
+    handleFirefoxArrowKeyNavigation(target, true, 'left');
+    expect(CursorUtils.setCursorPosition).toHaveBeenCalledWith(target, 0, 4);
+  });
+});

src/web/utils/firefoxUtils.ts

@@ -0,0 +1,40 @@
+import type {MarkdownTextInputElement} from '../../MarkdownTextInput.web';
+import {getCurrentCursorPosition, setCursorPosition} from './cursorUtils';
+
+/**
+ * Ensures consistent arrow navigation across grapheme clusters (like emojis)
+ */
+function handleFirefoxArrowKeyNavigation(target: MarkdownTextInputElement, isSelectionEvent = false, direction: 'right' | 'left' = 'right'): void {
+  const currentSelection = getCurrentCursorPosition(target);
+  if (!currentSelection) {
+    return;
+  }
+
+  const text = target.value;
+
+  const segmenter = new Intl.Segmenter('en', {granularity: 'grapheme'});
+  const graphemes = Array.from(segmenter.segment(text));
+
+  if (direction === 'right') {
+    const cursorPos = currentSelection.end;
+    const nextGrapheme = graphemes.find(({index, segment}) => {
+      const segmentEnd = index + segment.length;
+      return cursorPos < segmentEnd;
+    });
+
+    const newCursorPos = nextGrapheme ? nextGrapheme.index + nextGrapheme.segment.length : text.length;
+    setCursorPosition(target, isSelectionEvent ? currentSelection.start : newCursorPos, newCursorPos);
+  } else {
+    const cursorPos = currentSelection.start;
+    const prevGrapheme = graphemes.findLast(({index, segment}) => {
+      const segmentEnd = index + segment.length;
+      return segmentEnd < cursorPos;
+    });
+
+    const newCursorPos = prevGrapheme ? prevGrapheme.index + prevGrapheme.segment.length : 0;
+    setCursorPosition(target, newCursorPos, isSelectionEvent ? currentSelection.end : newCursorPos);
+  }
+}
+
+// eslint-disable-next-line import/prefer-default-export
+export {handleFirefoxArrowKeyNavigation};