Fix descenders cutoff in TextField (#26309)

* Include caret gaps in position calculation to avoid descender cutoff

* Calculate caret offset instead of exposing it

* Test that textfields scroll vertically by full line height

* Only add offset to end of caret to avoid going above 0.0

* Fix and test horizontal vs. vertical scrolling

* CR: docs and top caret offset logic

* Take max instead of assuming caretRect.top is always a full line down

* Always subtract caretOffset from caretTop since caretRect.top can be negative

packages/flutter/lib/src/widgets/editable_text.dart

@@ -752,8 +752,21 @@ class EditableTextState extends State<EditableText> with AutomaticKeepAliveClien
 
   // Calculate the new scroll offset so the cursor remains visible.
   double _getScrollOffsetForCaret(Rect caretRect) {
-    final double caretStart = _isMultiline ? caretRect.top : caretRect.left;
-    final double caretEnd = _isMultiline ? caretRect.bottom : caretRect.right;
+    double caretStart;
+    double caretEnd;
+    if (_isMultiline) {
+      // The caret is vertically centered within the line. Expand the caret's
+      // height so that it spans the line because we're going to ensure that the entire
+      // expanded caret is scrolled into view.
+      final double lineHeight = renderEditable.preferredLineHeight;
+      final double caretOffset = (lineHeight - caretRect.height) / 2;
+      caretStart = caretRect.top - caretOffset;
+      caretEnd = caretRect.bottom + caretOffset;
+    } else {
+      caretStart = caretRect.left;
+      caretEnd = caretRect.right;
+    }
+
     double scrollOffset = _scrollController.offset;
     final double viewportExtent = _scrollController.position.viewportDimension;
     if (caretStart < 0.0) // cursor before start of bounds

packages/flutter/test/material/text_field_test.dart

@@ -1722,7 +1722,42 @@ void main() {
     await skipPastScrollingAnimation(tester);
 
     scrollableState = tester.firstState(find.byType(Scrollable));
-    expect(scrollableState.position.pixels, isNot(equals(0.0)));
+    // For a horizontal input, scrolls to the exact position of the caret.
+    expect(scrollableState.position.pixels, equals(222.0));
+  });
+
+  testWidgets('Multiline text field scrolls the caret into view', (WidgetTester tester) async {
+    final TextEditingController controller = TextEditingController();
+
+    await tester.pumpWidget(
+      overlay(
+        child: Container(
+          child: TextField(
+            controller: controller,
+            maxLines: 6,
+          ),
+        ),
+      ),
+    );
+
+    const String tallText = 'a\nb\nc\nd\ne\nf\ng'; // One line over max
+    await tester.enterText(find.byType(TextField), tallText);
+    await skipPastScrollingAnimation(tester);
+
+    ScrollableState scrollableState = tester.firstState(find.byType(Scrollable));
+    expect(scrollableState.position.pixels, equals(0.0));
+
+    // Move the caret to the end of the text and check that the text field
+    // scrolls to make the caret visible.
+    controller.selection = const TextSelection.collapsed(offset: tallText.length);
+    await tester.pump();
+    await skipPastScrollingAnimation(tester);
+
+    // Should have scrolled down exactly one line height (7 lines of text in 6
+    // line text field).
+    final double lineHeight = findRenderEditable(tester).preferredLineHeight;
+    scrollableState = tester.firstState(find.byType(Scrollable));
+    expect(scrollableState.position.pixels, equals(lineHeight));
   });
 
   testWidgets('haptic feedback', (WidgetTester tester) async {