RangeSelector StepFrequency doesn't behave properly at larger values

[michael-hawker]

Describe the bug

In the sample app, with the example range between 0-100, values over 30 (40, 50) etc... don't act properly.

For instance, 30 snaps at 30 and 60 but not 90.
40 snaps at 40, but not 80
60 doesn't snap...

Steps to Reproduce

Steps to reproduce the behavior:

1. Go to RangeSelector sample in sample app.
2. Go to XAML tab
3. Set StepFrequency to a custom value
4. See behavior above

Expected behavior

30 should still snap at 30, 60, 90
40 should snap at 40 & 80
60 should snap at 60

Environment

Sample App

Windows 10 Build Number:
- [ ] Fall Creators Update (16299)
- [ ] April 2018 Update (17134)
- [ ] October 2018 Update (17763)
- [ ] May 2019 Update (18362)
- [x] May 2020 Update (19041)
- [ ] Insider Build (build number: )

Device form factor:
- [x] Desktop
- [ ] Xbox
- [ ] Surface Hub
- [ ] IoT

Additional context

Found when investigating #3473, FYI @hansmbakker

[skendrot]

I took a look and this is happening because of this line

            else if (newValue > Maximum || Maximum - newValue < StepFrequency)
            {
                return Maximum;
            }

For large values performing the Maximum - newValue will be less than StepFrequency.
Example: 100 - 90 = 10 which is less than 30.
I removed the or for the else statement and the values go up properly but then we see another issue. The max thumb doesn't calculate properly. Instead of going from 100 -> 70 -> 40 -> 10 it goes 100 -> 90 -> 60 -> 30. So we would need a new calculation method for finding the max thumb value as well.