[WASM][Canvas] Vertical slider button missing when slider is at the top

[StewartPolsky]

Describe the bug

Vertical slider button missing when slider is at the top

Steps to reproduce the bug

1. Go to https://xamlcontrolsgallery.platform.uno/
2. Click on Layout
3. Click on Canvas
4. Move vertical slider up to the top end
5. Notice that slider button is no longer visible

Expected behavior

Slider button should not exceed slider boundary

Actual behavior

Slider button exceeds slider boundary

Screenshots

Version Info

Gallery version (found on Settings page):

0.1.0-uno.1+246

Windows 10 version:

• [ ] Insider Build (xxxxx)
• [ ] October 2018 Update (17763)
• [ ] April 2018 Update (17134)
• [ ] Fall Creators Update (16299)
• [ ] Creators Update (15063)
• [ ] Anniversary Update (14393)
• [x] 1909 Build 18363.836

Device form factor:

• [x] Desktop
• [ ] Mobile
• [ ] Xbox
• [ ] Surface Hub
• [ ] IoT

Additional context

likely related to #68