[slifin]

Cursive has a repl powered debugger just put your breakpoint there use the repl to trigger the call then use intellji's debugger to step through

If you want to run expressions in context just use the expression editor like a normal repl

Something I haven't tried yet is redefining functions at debug time can't see why it wouldn't work though

[notriskfree]

Smalltalk famously let you code inside the debugger. You can run code with methods that don't exist yet and add them in the debugger when it notices they are missing.