add smooth-scrolling
Squashed 'modes/smooth-scrolling/' content from commit 0d9b228f952c

git-subtree-dir: modes/smooth-scrolling
git-subtree-split: 0d9b228f952c53ad456f98e2c761dda70ed72174
Merge commit 'e09d321971ad5ceaf33c8be93bcc1a55da1f62bd' as 'modes/smooth-scrolling'
Use smooth-scroll