feat: #953 improve arrow keys handler

Author: LucasXu0

frontend/app_flowy/packages/appflowy_editor/lib/src/document/selection.dart

@@ -46,9 +46,9 @@ class Selection {
       (start.path <= end.path && !pathEquals(start.path, end.path)) ||
       (isSingle && start.offset < end.offset);
 
-  Selection normalize() {
+  Selection get normalize {
     if (isForward) {
-      return Selection(start: end, end: start);
+      return reversed;
     }
     return this;
   }

frontend/app_flowy/packages/appflowy_editor/lib/src/render/rich_text/default_selectable.dart

@@ -32,7 +32,8 @@ mixin DefaultSelectable {
   Selection getSelectionInRange(Offset start, Offset end) =>
       forward.getSelectionInRange(start, end);
 
-  Offset localToGlobal(Offset offset) => forward.localToGlobal(offset);
+  Offset localToGlobal(Offset offset) =>
+      forward.localToGlobal(offset) - baseOffset;
 
   Selection? getWorldBoundaryInOffset(Offset offset) =>
       forward.getWorldBoundaryInOffset(offset);

frontend/app_flowy/packages/appflowy_editor/lib/src/render/rich_text/flowy_rich_text.dart

@@ -132,17 +132,24 @@ class _FlowyRichTextState extends State<FlowyRichText> with Selectable {
 
   @override
   List<Rect> getRectsInSelection(Selection selection) {
-    assert(pathEquals(selection.start.path, selection.end.path) &&
+    assert(selection.isSingle &&
         pathEquals(selection.start.path, widget.textNode.path));
 
     final textSelection = TextSelection(
       baseOffset: selection.start.offset,
       extentOffset: selection.end.offset,
     );
-    return _renderParagraph
+    final rects = _renderParagraph
         .getBoxesForSelection(textSelection, boxHeightStyle: BoxHeightStyle.max)
         .map((box) => box.toRect())
-        .toList();
+        .toList(growable: false);
+    if (rects.isEmpty) {
+      // If the rich text widget does not contain any text,
+      // there will be no selection boxes,
+      // so we need to return to the default selection.
+      return [Rect.fromLTWH(0, 0, 0, _renderParagraph.size.height)];
+    }
+    return rects;
   }
 
   @override