Add param threshold to isIntersectingViewport

[ron0115]

When I write an e2e test about controlling scrollbar, I found I got visibleRatio = 0.001xxxx，I must add a threshold to finish this judge in my situation. Looking forward to your comments

[google-cla]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed (or fixed any issues), please reply here with @googlebot I signed it! and we'll verify it.

---

What to do if you already signed the CLA

Individual signers

• It's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.

Corporate signers

• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
• The email used to register you as an authorized contributor must be the email used for the Git commit. Check your existing CLA data and verify that your email is set on your git commits.
• The email used to register you as an authorized contributor must also be attached to your GitHub account.

ℹ️ Googlers: Go here for more info.

[ron0115]

@googlebot I signed it!

[mathiasbynens]

Since this is an API change, could you please add a test?

[jackfranklin]

I quite like this change. I wonder about the API being { threshold: X } (so it's self-documenting & easy to add more options in future).

@mathiasbynens wdyt about this?

[mathiasbynens]

Agreed on using an options bag.

[ron0115]

@mathiasbynens I add a unit test about isIntersectingViewport~Thanks

[ron0115]

I couldn't handle this pr,colud anyone please resolve the merging conflicts? @jackfranklin

[google-cla]

All (the pull request submitter and all commit authors) CLAs are signed, but one or more commits were authored or co-authored by someone other than the pull request submitter.

We need to confirm that all authors are ok with their commits being contributed to this project. Please have them confirm that by leaving a comment that contains only @googlebot I consent. in this pull request.

Note to project maintainer: There may be cases where the author cannot leave a comment, or the comment is not properly detected as consent. In those cases, you can manually confirm consent of the commit author(s), and set the cla label to yes (if enabled on your project).

ℹ️ Googlers: Go here for more info.

[jschfflr]

@googlebot I consent.

[jschfflr]

@ron0115 I went ahead and resolved the merge conflicts while also updating the implementation a bit. Let me know if you're ok with the changes and I'm happy to get this landed :)