fix(demo/barstack): improve Tooltip positioning logic

[williaster]

📝 Documentation

🐛 Bug Fix

This closes #1016 and improves the logic for tooltip positioning in the /barstack demo. That example was refactored to use TooltipInPortal when it was added, and the logic wasn't as robust as it should have been.

tl;dr left/top should have been in SVG coordinates, but top was in viewport coordinates. If anything is rendered above the chart this breaks (as shown in the issue, reproduced with the yellow box below).

Before

After

@hshoff
cc @wisyr

[coveralls]

Pull Request Test Coverage Report for Build 479261526

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 61.336%

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Totals
Change from base Build 478565296:	0.0%
Covered Lines:	1737
Relevant Lines:	2699

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 479261526

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 61.336%

---

Totals
Change from base Build 478565296:	0.0%
Covered Lines:	1737
Relevant Lines:	2699

---

💛 - Coveralls