Change the overflow property from scroll to auto

[ProudBenzene]

📕 Description

Please include a summary of the change and which issue is fixed. Please also include relevant motivation and context. List any dependencies that are required for this change.

Please list all the possible issues that would be covered in this Pull Request

Fixes #253

What is the current behavior?

What is the new behavior?

Other Information or Things to be taken Note of

I got trouble with the same issue in #253 , and as far as I know, it seems that there was no successful reproduction on your end.

Here I made some debug about the phenomenon, and it seems that the overflow of the scrollbars of mathblocks is to blame.

When I set the property to scroll, the scrollbars would always be displayed, regardless of whether it contained an actual "bar"(like gif1) – and that's why the white stripes were formed; and when I set the property to auto, the scrollbar appeared only below long formulas(like gif2), instead of all the formulas like before. So the issue #253 was solved completely.

🔖 Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

✅ Checklist

All checkboxes below must be followed and checked.

• I followed the Developer's Guide when building for Primary

Primary uses Sass and Grunt so building the project is important to ensure the final production code is up to date with changes from all the .scss and .css files. If you are unsure about this please look into the Developer's section inside the README or if you have any questions, post an issue or send us a message before sending in this Pull Request.

• My code follows the style guidelines of Primary
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings