Fix Scaffold extend body (#157441)

Fixes: #157316

packages/flutter/lib/src/material/scaffold.dart

@@ -1087,17 +1087,21 @@ class _ScaffoldLayout extends MultiChildLayoutDelegate {
     if (hasChild(_ScaffoldSlot.body)) {
       double bodyMaxHeight = math.max(0.0, contentBottom - contentTop);
 
-      if (extendBody) {
+      // When extendBody is true, the body is visible underneath the bottom widgets.
+      // This does not apply when the area is obscured by the device keyboard.
+      if (extendBody && minInsets.bottom <= bottomWidgetsHeight) {
         bodyMaxHeight += bottomWidgetsHeight;
         bodyMaxHeight = clampDouble(bodyMaxHeight, 0.0, looseConstraints.maxHeight - contentTop);
         assert(bodyMaxHeight <= math.max(0.0, looseConstraints.maxHeight - contentTop));
+      } else {
+        bottomWidgetsHeight = 0.0;
       }
 
       final BoxConstraints bodyConstraints = _BodyBoxConstraints(
         maxWidth: fullWidthConstraints.maxWidth,
         maxHeight: bodyMaxHeight,
         materialBannerHeight: materialBannerSize.height,
-        bottomWidgetsHeight: extendBody ? bottomWidgetsHeight : 0.0,
+        bottomWidgetsHeight: bottomWidgetsHeight,
         appBarHeight: appBarHeight,
       );
       layoutChild(_ScaffoldSlot.body, bodyConstraints);
@@ -3086,9 +3090,6 @@ class ScaffoldState extends State<Scaffold> with TickerProviderStateMixin, Resto
       bottom: _resizeToAvoidBottomInset && MediaQuery.viewInsetsOf(context).bottom != 0.0 ? 0.0 : null,
     );
 
-    // extendBody locked when keyboard is open
-    final bool extendBody = minInsets.bottom <= 0 && widget.extendBody;
-
     return _ScaffoldScope(
       hasDrawer: hasDrawer,
       geometryNotifier: _geometryNotifier,
@@ -3102,7 +3103,7 @@ class ScaffoldState extends State<Scaffold> with TickerProviderStateMixin, Resto
               },
               child: CustomMultiChildLayout(
                 delegate: _ScaffoldLayout(
-                  extendBody: extendBody,
+                  extendBody: widget.extendBody,
                   extendBodyBehindAppBar: widget.extendBodyBehindAppBar,
                   minInsets: minInsets,
                   minViewPadding: minViewPadding,

packages/flutter/test/material/scaffold_test.dart

@@ -3418,6 +3418,36 @@ void main() {
     ));
     expect(scaffoldMaterial.color, theme.colorScheme.surface);
   });
+
+  testWidgets('Body height remains Scaffold height when keyboard is smaller than bottomNavigationBar and extendBody is true', (WidgetTester tester) async {
+    final Key bodyKey = UniqueKey();
+    Widget buildFrame({double keyboardHeight = 0}) {
+      return MaterialApp(
+        home: Builder(
+          builder: (BuildContext context) {
+            return MediaQuery(
+              data: MediaQuery.of(context).copyWith(
+                viewInsets: EdgeInsets.only(bottom: keyboardHeight),
+              ),
+              child: Scaffold(
+                extendBody: true,
+                body: SizedBox.expand(key: bodyKey),
+                bottomNavigationBar:const SizedBox(height: 100),
+              ),
+            );
+          },
+        ),
+      );
+    }
+    await tester.pumpWidget(buildFrame());
+    expect(tester.getSize(find.byKey(bodyKey)).height, 600);
+
+    await tester.pumpWidget(buildFrame(keyboardHeight: 100));
+    expect(tester.getSize(find.byKey(bodyKey)).height, 600);
+
+    await tester.pumpWidget(buildFrame(keyboardHeight: 200));
+    expect(tester.getSize(find.byKey(bodyKey)).height, 400);
+  });
 }
 
 class _GeometryListener extends StatefulWidget {