It looks that "maxArgs" should be set to 1 for devtools.elements.createSidebarPane:

• if you look to the api docs for devtools.panels.create in the chrome.devtools.panels doc page you can see that it does have 3 mandatory parameters + the optional callback
• the api docs for devtools.panels.elements.createSidebarPane in the same doc page shows that it does have 1 mandatory parameter (the sidebar title) + the optional callback

and so maxArgs should be set to the maximum number of parameters that the API method does expect without counting the optional callback.

The reason for that is that the corresponding promised-based API method signature shouldn't expect a callback parameter (and the polyfill will pass a callback internally to the wrapped chrome callback-based API method), e.g.:

chrome.devtools.panels.elements.createSidebarPane("my sidebar", (sidebarPanel) => ...);

translates into:

const sidebarPanel = await browser.devtools.panels.elements.createSidebarPane("my sidebar");
...
// or
devtools.panels.elements.createSidebarPane("my sidebar").then(sidebarPanel => ...);

Thanks again for the explanation. That makes a lot of sense.
Could you also explain what the role of singleCallbackArg property is?
I looked at the following lines and I am a little unsure about when to use this property.

For context, see #168 (comment)