This is kind of crazy… Here’s a GIF of me bolding the voltage symbol in Atlassian Confluence's wiki editor:
It seems that bolding this symbol ⚡︎ results in the display of a completely different symbol (which looks like ➫) instead. I don’t think it actually changes it, because copying the arrow and pasting it elsewhere shows the same old voltage symbol.
Confluence's font stack appears to be -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Noto Sans", Ubuntu, "Droid Sans", "Helvetica Neue", sans-serif.
What is producing this result? It seems like the kind of thing that could happen with a bad web font, but that’s a system font stack.
It seems that Stack Overflow suffers the same exact problem:
Voltage symbol: ⚡︎
Voltage symbol, bolded: ⚡︎
And here’s what I see:
I’m browsing with Safari on OS X High Sierra 10.13.6. Notably, I cannot replicate this in e.g. the Notes app, even though that uses the system font (San Francisco).
Using Chrome just shows these missing-character symbols instead:
And things look just fine in Firefox:
What gives?
Update (2018-08-03): I just checked in Safari Technical Preview (their dev/preview release, similar to Chrome Canary), and this bug is no longer reproducible. Good, that means I don’t have to file a WebKit bug today.
Update (2018-09-05): I can no longer replicate in Safari I suppose I am still curious what may have caused this bug, so I will leave the question open for any compelling theories or for an Apple engineer to confirm.