Use baseline value to get position in next line (#93129)

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

@@ -161,16 +161,6 @@ class VerticalCaretMovementRun extends BidirectionalIterator<TextPosition> {
     return _isValid;
   }
 
-  // Computes the vertical distance from the `from` line's bottom to the `to`
-  // lines top.
-  double _lineDistance(int from, int to) {
-    double lineHeight = 0;
-    for (int index = from + 1; index < to; index += 1) {
-      lineHeight += _lineMetrics[index].height;
-    }
-    return lineHeight;
-  }
-
   final Map<int, MapEntry<Offset, TextPosition>> _positionCache = <int, MapEntry<Offset, TextPosition>>{};
 
   MapEntry<Offset, TextPosition> _getTextPositionForLine(int lineNumber) {
@@ -181,11 +171,8 @@ class VerticalCaretMovementRun extends BidirectionalIterator<TextPosition> {
       return cachedPosition;
     }
     assert(lineNumber != _currentLine);
-    final double distanceY = lineNumber > _currentLine
-      ? _lineMetrics[_currentLine].descent + _lineMetrics[lineNumber].ascent + _lineDistance(_currentLine, lineNumber)
-      : - _lineMetrics[_currentLine].ascent - _lineMetrics[lineNumber].descent - _lineDistance(lineNumber, _currentLine);
 
-    final Offset newOffset = _currentOffset.translate(0, distanceY);
+    final Offset newOffset = Offset(_currentOffset.dx, _lineMetrics[lineNumber].baseline);
     final TextPosition closestPosition = _editable._textPainter.getPositionForOffset(newOffset);
     final MapEntry<Offset, TextPosition> position = MapEntry<Offset, TextPosition>(newOffset, closestPosition);
     _positionCache[lineNumber] = position;
@@ -2418,17 +2405,16 @@ class RenderEditable extends RenderBox with RelayoutWhenSystemFontsChangeMixin,
     // TODO(LongCatIsLooong): include line boundaries information in
     // ui.LineMetrics, then we can get rid of this.
     final Offset offset = _textPainter.getOffsetForCaret(startPosition, Rect.zero);
-    int line = 0;
-    double accumulatedHeight = 0;
     for (final ui.LineMetrics lineMetrics in metrics) {
-      if (accumulatedHeight + lineMetrics.height > offset.dy) {
-        return MapEntry<int, Offset>(line, Offset(offset.dx, lineMetrics.baseline));
+      if (lineMetrics.baseline + lineMetrics.descent > offset.dy) {
+        return MapEntry<int, Offset>(lineMetrics.lineNumber, Offset(offset.dx, lineMetrics.baseline));
       }
-      line += 1;
-      accumulatedHeight += lineMetrics.height;
     }
     assert(false, 'unable to find the line for $startPosition');
-    return MapEntry<int, Offset>(math.max(0, metrics.length - 1), Offset(offset.dx, accumulatedHeight));
+    return MapEntry<int, Offset>(
+      math.max(0, metrics.length - 1),
+      Offset(offset.dx, metrics.isNotEmpty ? metrics.last.baseline + metrics.last.descent : 0.0),
+    );
   }
 
   /// Starts a [VerticalCaretMovementRun] at the given location in the text, for

packages/flutter/test/widgets/editable_text_shortcuts_tests.dart

@@ -57,7 +57,12 @@ void main() {
   final TextEditingController controller = TextEditingController(text: testText);
 
   final FocusNode focusNode = FocusNode();
-  Widget buildEditableText({ TextAlign textAlign = TextAlign.left, bool readOnly = false, bool obscured = false }) {
+  Widget buildEditableText({
+    TextAlign textAlign = TextAlign.left,
+    bool readOnly = false,
+    bool obscured = false,
+    TextStyle style = const TextStyle(fontSize: 10.0),
+  }) {
     return MaterialApp(
       home: Align(
         alignment: Alignment.topLeft,
@@ -69,7 +74,7 @@ void main() {
             showSelectionHandles: true,
             autofocus: true,
             focusNode: focusNode,
-            style: const TextStyle(fontSize: 10),
+            style: style,
             textScaleFactor: 1,
             // Avoid the cursor from taking up width.
             cursorWidth: 0,
@@ -1593,6 +1598,32 @@ void main() {
               offset: 3,   // Would have been 4 if the run wasn't interrupted.
             ));
           }, variant: TargetPlatformVariant.all());
+
+          testWidgets('long run with fractional text height', (WidgetTester tester) async {
+            controller.text = "${'źdźbło\n' * 49}źdźbło";
+            controller.selection = const TextSelection.collapsed(offset: 2);
+            await tester.pumpWidget(buildEditableText(style: const TextStyle(fontSize: 13.0, height: 1.17)));
+
+            for (int i = 1; i <= 49; i++) {
+              await sendKeyCombination(tester, const SingleActivator(LogicalKeyboardKey.arrowDown));
+              await tester.pump();
+              expect(
+                controller.selection,
+                TextSelection.collapsed(offset: 2 + i * 7),
+                reason: 'line $i',
+              );
+            }
+
+            for (int i = 49; i >= 1; i--) {
+              await sendKeyCombination(tester, const SingleActivator(LogicalKeyboardKey.arrowUp));
+              await tester.pump();
+              expect(
+                controller.selection,
+                TextSelection.collapsed(offset: 2 + (i - 1) * 7),
+                reason: 'line $i',
+              );
+            }
+          }, variant: TargetPlatformVariant.all());
         });
       });
     },