Using "%s" as an argument name causes unexpected behavior

[thisandagain]

Expected Behavior

• Creating a custom procedure with "%s" as an argument name should work

Actual Behavior

• Causes a number of exceptions that lead to instability

Steps to Reproduce

• Click the "Make a block" button
• Enter %s as the argument name
• Click "ok"

Demo

Operating System and Browser

Mac OS 10.13.2 | Chrome Version 63.0.3239.132 (Official Build) (64-bit)

---

Moved from scratchfoundation/scratch-gui#1347
/cc @thisandagain @Kenny2github @amazinigmech2418

[rachel-fenichel]

From a quick look through code, I think the solution is proper escaping in this function:
https://github.com/LLK/scratch-blocks/blob/develop/blocks_vertical/procedures.js#L529

[Kenny2github]

@thisandagain might wanna add the "custom procedures" label?

[thisandagain]

Thanks @Kenny2github. Also added "help wanted" to this.

[apple502j]

The same as "%m" - JSON opCode?

[AmazingMech2418]

I think so @apple502j . Maybe a filter is needed to stop the program from reading it as JSON.