A few debugging questions:

- What Skuid version are you running?
- Sandbox or Production?
- Is My Domain enabled in this org?
- Do you have the Critical Update “Serve Static Resources from the Visualforce Domain” activated?

• Skuid version : 7.24
  - Sandbox
  - My Domain is enabled
  - That critical update is NOT activated
  
  

Can you try activating that critical update?

Also, open the JavaScript Console in Chrome, and do a “hard reload and empty cache” — once the console is open, press and hold on the “refresh” button in Chrome’s toolbar and an option will appear to “Hard reload and Empty Cache” — press this.

Activating the critical update fixed everything!! Also I believe I once posted requesting the ability to add custom components to the “Lightning Component” component in skuid (that’s a lot of uses of the word component). Apparently , although it doesn’t show up in the dropdown, you can still hack the XML to show your component! This line:

<lightning lightningid="markup://c:ExcelUpload"/>

is working awesomely! Thanks Zach.

For posterity, the final solution here is to go to Setup > Critical Updates, and Activate the “Serve Static Resources from the Visualforce Domain” Critical Update.

That’s true - it will work just fine in runtime, you just can’t currently choose it from the Page Composer.