Normally, after adding a reaction, it highlights in blue:
In some situations, it doesn't get highlighted:
This is because templates aren't reloaded in postToggleReactionHandler, so containsCurrentUser and reactionTooltip template functions weren't updated for current user.
It was only happening when more than 1 user was using issuesapp, that's why it wasn't 100% reproducible.
Fixed in https://github.com/shurcooL/issuesapp/commit/6e0dfc682815f0e381f474ac860f23369ab8efc2.