CT: interaction is flakey on Safari

[pixelzoom]

CT has the following interaction problems on Safari. As of this writing, I'm using macOS 10.15.3 + Safari 13.0.5 (15608.5.11). But they've been problems for as long as I can remember. I just finally got frustrated enough to report them.

• Clicking on a yellow/red cell in the grid does not reliably open the details popup. It often takes several clicks. When this occurs, there's usually no hand cursor when mousing over the cells.

• Clicking outside of the detail popup does not reliably close it. It often takes several attempts.

• After clicking "Open in new tab" in the details popup, it often takes multiple attempts to select and copy text in that new tab.

Assigning to @ariel-phet to prioritize and assign.

[ariel-phet]

Will plan to discuss when #84 gets discussed. It seems like after ES6 migration we should dedicate a bit of time to some CT enhancements