A user reported a jittering pop-up issue when selecting text and hovering over the “Share” option, which causes the pop-up to rapidly appear and disappear due to the text size and hover logic. dfabulich explained the technical cause, and JammyDodger confirmed the issue and forwarded it to the team. awesomerobot later confirmed the fix was implemented in a recent update, with a screenshot showing the resolved behavior.
To reproduce:
On a machine with a mouse pointer or trackpad, select a word in this post. You’ll see a pop-up with options to “Quote” or “Share.” When you hover over “Share,” the popup wiggles a bit as it changes to a tweet and an email button.
Now, slowly approach the pop-up from the right side. The Share pop-up will jitter rapidly.
This is happening because the “Share” text is larger than the share buttons, so when your mouse curser is at the right edge of the Share button, the Share button disappears to be replaced with sharing options, shortening the pop-up, but now your mouse cursor is no longer hovering over the sharing options, which dismisses the sharing options.
Thus the Share button is constantly appearing and disappearing, creating the jitter effect.
I can replicate this on here. 
That’s a fun one. 
I’ll pass it on to the relevant team and see if they can give it a tweak.
I’ve merged in some updates to the quote popup that should fix this issue!
https://github.com/discourse/discourse/commit/3483285b89c72bec0f09d1b4dc9d2456259e70ba
