Explicitly set affinity when moving left/right by line (#87133)

packages/flutter/lib/src/rendering/editable.dart

@@ -1965,6 +1965,7 @@ class RenderEditable extends RenderBox with RelayoutWhenSystemFontsChangeMixin,
     final TextSelection selectedLine = _getLineAtOffset(TextPosition(offset: startPoint));
     final TextSelection nextSelection = TextSelection.collapsed(
       offset: selectedLine.baseOffset,
+      affinity: TextAffinity.downstream,
     );
 
     _setSelection(nextSelection, cause);
@@ -2051,6 +2052,7 @@ class RenderEditable extends RenderBox with RelayoutWhenSystemFontsChangeMixin,
     final TextSelection selectedLine = _getLineAtOffset(TextPosition(offset: startPoint));
     final TextSelection nextSelection = TextSelection.collapsed(
       offset: selectedLine.extentOffset,
+      affinity: TextAffinity.upstream,
     );
 
     _setSelection(nextSelection, cause);

packages/flutter/test/rendering/editable_test.dart

@@ -925,6 +925,7 @@ void main() {
     editable.moveSelectionRightByLine(SelectionChangedCause.keyboard);
     expect(currentSelection.isCollapsed, true);
     expect(currentSelection.baseOffset, 13);
+    expect(currentSelection.affinity, TextAffinity.upstream);
     // RenderEditable relies on its parent that passes onSelectionChanged to set
     // the selection.
 
@@ -933,21 +934,25 @@ void main() {
     editable.moveSelectionRightByLine(SelectionChangedCause.keyboard);
     expect(currentSelection.isCollapsed, true);
     expect(currentSelection.baseOffset, 13);
+    expect(currentSelection.affinity, TextAffinity.upstream);
 
     // Move back to the start of the line.
     editable.moveSelectionLeftByLine(SelectionChangedCause.keyboard);
     expect(currentSelection.isCollapsed, true);
     expect(currentSelection.baseOffset, 0);
+    expect(currentSelection.affinity, TextAffinity.downstream);
 
     // Trying moveSelectionLeftByLine does nothing at the leftmost of the field.
     editable.moveSelectionLeftByLine(SelectionChangedCause.keyboard);
     expect(currentSelection.isCollapsed, true);
     expect(currentSelection.baseOffset, 0);
+    expect(currentSelection.affinity, TextAffinity.downstream);
 
     // Move the selection to the empty line.
     editable.moveSelectionRightByLine(SelectionChangedCause.keyboard);
     expect(currentSelection.isCollapsed, true);
     expect(currentSelection.baseOffset, 13);
+    expect(currentSelection.affinity, TextAffinity.upstream);
     editable.moveSelectionRight(SelectionChangedCause.keyboard);
     expect(currentSelection.isCollapsed, true);
     expect(currentSelection.baseOffset, 14);
@@ -957,9 +962,11 @@ void main() {
     editable.moveSelectionLeftByLine(SelectionChangedCause.keyboard);
     expect(currentSelection.isCollapsed, true);
     expect(currentSelection.baseOffset, 14);
+    expect(currentSelection.affinity, TextAffinity.downstream);
     editable.moveSelectionRightByLine(SelectionChangedCause.keyboard);
     expect(currentSelection.isCollapsed, true);
     expect(currentSelection.baseOffset, 14);
+    expect(currentSelection.affinity, TextAffinity.downstream);
   }, skip: isBrowser); // https://github.com/flutter/flutter/issues/61021
 
   test('arrow keys and delete handle simple text correctly', () async {