[Material] Unit test for skipping Slider tick mark due to overdensity (#28013)

Added a unit test to check that the tick marks are skipped over when the divisions are set to a number that makes them less than 6dp away from each other.

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

diff --git a/packages/flutter/lib/src/material/slider.dart b/packages/flutter/lib/src/material/slider.dart
index 9408bba..50f12cc 100644
--- a/packages/flutter/lib/src/material/slider.dart
+++ b/packages/flutter/lib/src/material/slider.dart
@@ -1005,16 +1005,16 @@
         isEnabled: isInteractive,
         sliderTheme: _sliderTheme,
       ).width;
-      // If the ticks would be too dense, don't bother painting them.
-      if ((trackRect.width - tickMarkWidth) / divisions >= 3.0 * tickMarkWidth) {
+      final double adjustedTrackWidth = trackRect.width - tickMarkWidth;
+      // If the tick marks would be too dense, don't bother painting them.
+      if (adjustedTrackWidth / divisions >= 3.0 * tickMarkWidth) {
+        final double dy = trackRect.center.dy;
         for (int i = 0; i <= divisions; i++) {
-          final double tickValue = i / divisions;
+          final double value = i / divisions;
           // The ticks are mapped to be within the track, so the tick mark width
           // must be subtracted from the track width.
-          final double tickX = trackRect.left +
-              tickValue * (trackRect.width - tickMarkWidth) + tickMarkWidth / 2;
-          final double tickY = trackRect.center.dy;
-          final Offset tickMarkOffset = Offset(tickX, tickY);
+          final double dx = trackRect.left + value * adjustedTrackWidth + tickMarkWidth / 2;
+          final Offset tickMarkOffset = Offset(dx, dy);
           _sliderTheme.tickMarkShape.paint(
             context,
             tickMarkOffset,

packages/flutter/test/material/slider_test.dart

diff --git a/packages/flutter/test/material/slider_test.dart b/packages/flutter/test/material/slider_test.dart
index 19f4d0b..3cf2a86 100644
--- a/packages/flutter/test/material/slider_test.dart
+++ b/packages/flutter/test/material/slider_test.dart
@@ -558,8 +558,8 @@
       final ValueChanged<double> onChanged = !enabled
           ? null
           : (double d) {
-              value = d;
-            };
+        value = d;
+      };
       return Directionality(
         textDirection: TextDirection.ltr,
         child: MediaQuery(
@@ -978,6 +978,55 @@
     await tester.pumpAndSettle();
   });
 
+  testWidgets('Tick marks are skipped when they are too dense', (WidgetTester tester) async {
+    Widget buildSlider({
+      int divisions,
+    }) {
+      return Directionality(
+        textDirection: TextDirection.ltr,
+        child: MediaQuery(
+          data: MediaQueryData.fromWindow(window),
+          child: Material(
+            child: Center(
+              child: Slider(
+                min: 0.0,
+                max: 100.0,
+                divisions: divisions,
+                value: 0.25,
+                onChanged: (double newValue) { },
+              ),
+            ),
+          ),
+        ),
+      );
+    }
+
+    // Pump a slider with a reasonable amount of divisions to verify that the
+    // tick marks are drawn when the number of tick marks is not too dense.
+    await tester.pumpWidget(
+      buildSlider(
+        divisions: 4,
+      ),
+    );
+
+    final RenderBox sliderBox = tester.firstRenderObject<RenderBox>(find.byType(Slider));
+
+    // 5 tick marks and a thumb.
+    expect(sliderBox, paintsExactlyCountTimes(#drawCircle, 6));
+
+    // 200 divisions will produce a tick interval off less than 6,
+    // which would be too dense to draw.
+    await tester.pumpWidget(
+      buildSlider(
+        divisions: 200,
+      ),
+    );
+
+    // No tick marks are drawn because they are too dense, but the thumb is
+    // still drawn.
+    expect(sliderBox, paintsExactlyCountTimes(#drawCircle, 1));
+  });
+
   testWidgets('Slider has correct animations when reparented', (WidgetTester tester) async {
     final Key sliderKey = GlobalKey(debugLabel: 'A');
     double value = 0.0;