[karenrasmussen]

Context

Kanopy rebuilt its frontend and now ships it as an ES module. The page loads through Via but never boots: the browser discards the bundle before running a line of it, leaving Kanopy's loading screen up forever. pywb wraps every script it serves in a block which redeclares window, location and the rest so wombat can proxy them.
A module rejects that wrapper twice over, it declares let arguments, which strict mode forbids, and it encloses the module's export in a block.
pywb has no notion of modules at all: every <script src> maps to the classic JS modifier whatever its type. This is a known pywb bug, open since 2022 (webrecorder/pywb#771 for the strict mode error, #964 for the enclosed export, #764 tracking the fix). 2.10.0 doesn't address it, so upgrading isn't a way out.

Changes

A modifier of our own, esm_, applied to <script type="module"> and <link rel="modulepreload">, and a rewriter which serves what carries it: pywb's own wrapper minus let arguments and minus the block. The block can go because a module's outermost scope is already its own, a classic script's is the global scope, which is why pywb needs one. Dropping it also leaves the export at the top level, where it must be.