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Thanks for the contribution!

Can you add a test for this?

CC @clocksmith

LGTM, but please add a test

I'm sorry but I don't have time to write a test, if someone would like to do this fix and write a test I'd be happy to close this PR and let that one merge.

Unfortunately we can't merge a change like this without a test. I did a little digging to see what a test for this would look like, and it looks like this code passes today because all the implementations that touch these properties use sizes where height == width. So a test should create a property where the height > width, pass it into the theme, and make sure the widget is laid out correctly.

  testWidgets('Slider respects height from theme', (WidgetTester tester) async {
    final Key sliderKey = UniqueKey();
    double value = 0.0;
    await tester.pumpWidget(
      Directionality(
        textDirection: TextDirection.ltr,
        child: StatefulBuilder(
          builder: (BuildContext context, StateSetter setState) {
            final SliderThemeData sliderTheme = SliderTheme.of(context).copyWith(tickMarkShape: TallSliderTickMarkShape());
            return MediaQuery(
              data: MediaQueryData.fromWindow(window),
              child: Material(
                child: Center(
                  child: IntrinsicHeight(
                    child: SliderTheme(
                      data: sliderTheme,
                      child: Slider(
                        key: sliderKey,
                        value: value,
                        divisions: 4,
                        onChanged: (double newValue) {
                          setState(() {
                            value = newValue;
                          });
                        },
                      ),
                    ),
                  ),
                ),
              ),
            );
          },
        ),
      ),
    );

    final RenderBox renderObject = tester.renderObject<RenderBox>(find.byType(Slider));
    expect(renderObject.size.height, 200);
  });

Where TallSliderTickMarkShape is something like:

class TallSliderTickMarkShape extends SliderTickMarkShape {
  @override
  Size getPreferredSize({SliderThemeData sliderTheme, bool isEnabled}) {
    return const Size(10.0, 200.0);
  }

  @override
  void paint(
      PaintingContext context,
      Offset offset, {
      Offset thumbCenter,
      RenderBox parentBox,
      SliderThemeData sliderTheme,
      Animation<double> enableAnimation,
      bool isEnabled,
      TextDirection textDirection,
    }) {
    final Paint paint = Paint()..color = Colors.red;
    context.canvas.drawRect(Rect.fromLTWH(offset.dx, offset.dy, 10.0, 20.0), paint);
  }
}

@dnfield Shouldn't this height > width case be for the whole Slider component (the inner RenderBox), not just the TickMarkShape?

@clocksmith - any one of those will cause the height to be higher