Fix InteractiveViewer minScale bug (#65432)

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

@@ -646,26 +646,21 @@ class _InteractiveViewerState extends State<InteractiveViewer> with TickerProvid
     // Don't allow a scale that results in an overall scale beyond min/max
     // scale.
     final double currentScale = _transformationController.value.getMaxScaleOnAxis();
-    final double totalScale = currentScale * scale;
+    final double totalScale = math.max(
+      currentScale * scale,
+      // Ensure that the scale cannot make the child so big that it can't fit
+      // inside the boundaries (in either direction).
+      math.max(
+        _viewport.width / _boundaryRect.width,
+        _viewport.height / _boundaryRect.height,
+      ),
+    );
     final double clampedTotalScale = totalScale.clamp(
       widget.minScale,
       widget.maxScale,
     ) as double;
     final double clampedScale = clampedTotalScale / currentScale;
-    final Matrix4 nextMatrix = matrix.clone()..scale(clampedScale);
-
-    // Ensure that the scale cannot make the child so big that it can't fit
-    // inside the boundaries (in either direction).
-    final double minScale = math.max(
-      _viewport.width / _boundaryRect.width,
-      _viewport.height / _boundaryRect.height,
-    );
-    if (clampedTotalScale < minScale) {
-      final double minCurrentScale = minScale / currentScale;
-      return matrix.clone()..scale(minCurrentScale);
-    }
-
-    return nextMatrix;
+    return matrix.clone()..scale(clampedScale);
   }
 
   // Return a new matrix representing the given matrix after applying the given

packages/flutter/test/widgets/interactive_viewer_test.dart

@@ -729,6 +729,47 @@ void main() {
       await tester.pumpAndSettle();
       expect(transformationController.value.getMaxScaleOnAxis(), greaterThan(1.0));
     });
+
+    // Regression test for https://github.com/flutter/flutter/issues/65304
+    testWidgets('can view beyond boundary when necessary for a small child', (WidgetTester tester) async {
+      final TransformationController transformationController = TransformationController();
+      await tester.pumpWidget(
+        MaterialApp(
+          home: Scaffold(
+            body: Center(
+              child: InteractiveViewer(
+                constrained: false,
+                minScale: 1.0,
+                maxScale: 1.0,
+                transformationController: transformationController,
+                child: const SizedBox(width: 200.0, height: 200.0),
+              ),
+            ),
+          ),
+        ),
+      );
+
+      expect(transformationController.value, equals(Matrix4.identity()));
+
+      // Pinch to zoom does nothing because minScale and maxScale are 1.0.
+      final Offset center = tester.getCenter(find.byType(SizedBox));
+      final Offset scaleStart1 = Offset(center.dx - 10.0, center.dy - 10.0);
+      final Offset scaleStart2 = Offset(center.dx + 10.0, center.dy + 10.0);
+      final Offset scaleEnd1 = Offset(center.dx - 20.0, center.dy - 20.0);
+      final Offset scaleEnd2 = Offset(center.dx + 20.0, center.dy + 20.0);
+      final TestGesture gesture = await tester.createGesture();
+      final TestGesture gesture2 = await tester.createGesture();
+      await gesture.down(scaleStart1);
+      await gesture2.down(scaleStart2);
+      await tester.pump();
+      await gesture.moveTo(scaleEnd1);
+      await gesture2.moveTo(scaleEnd2);
+      await tester.pump();
+      await gesture.up();
+      await gesture2.up();
+      await tester.pumpAndSettle();
+      expect(transformationController.value, equals(Matrix4.identity()));
+    });
   });
 
   group('getNearestPointOnLine', () {