Fans were quick to notice the two music stars briefly coming within feet of one another Sunday inside of Crypto.com Arena -- specifically, when Taylor was making her big entrance and getting a round of applause from everyone.

There's video and photos of this moment that have been circulating ... and you can see Tay Tay walking right past CH -- who was sitting at a table with his back turned to her.

In a clip of this, it appears Calvin saw Taylor but turned his head to avoid eye contact -- and this led fans to run wild with theories that Taylor saw him too ... and that there, perhaps, was a little awkwardness between them since they were both in the building and mingling.

Unfortunately for the gossip crowd, there's nothing to speculate on. Eyewitnesses who were there tell us Taylor and Calvin didn't talk -- nor did they have any kind of specific run-in that would require them to chat ... let alone acknowledge one another's presence.

We're told T-Swift and Calvin weren't even in the same space during the show -- outside of this one fleeting moment when she happened to pass by him -- and eyewitnesses insist ... he was preoccupied with his beautiful wife, Vick Hope, and was paying no mind to Taylor.

Lucky for him ... that feeling applies to Taylor too. Sources with direct knowledge tell us she didn't even register he was in attendance -- so he might as well have not been there at all.

Taylor and Calvin dated for a hot minute in 2015 and 2016 -- they were together for a little over a year ... and then their romance flamed out before they each moved on to the next.

Considering Taylor still seems to have it for some exes -- see the Joe Alwyn connection to her new album -- it makes sense why some would assume there was bad blood here.

That's just not the case though ... Calvin is in her rearview, and wasn't even invisible in her side-view mirrors for this brief reunion of sorts.