Chrome extension can not access custom window properties - google-chrome

Question

Im trying to write a Chrome extension that has a dev tools panel. This extension needs to call functions defined on a property on window in a webpage that I also have made. In other words, the extension is only for my own web page and I control both. Example:
// This script is added in my own webpage application when it loads
window.myTest = () => { /* do something */ }
I want to be able to call the function window.myTest from my Chrome extension. I need to make similar functionality like https://github.com/zalmoxisus/redux-devtools-extension.
It seems that I need to do this by inject script/code from my backend page. I have all working, extension with backend page that gets invoked and I can see that the code that I inject gets called in the page context (testing by console.log gets written to the console output of the page).
Here is my code:
manifest.json
{
"manifest_version": 2,
"name": "MyTest",
"description": "MyTest",
"version": "0.0.1",
"minimum_chrome_version": "10.0",
"devtools_page": "devtools.html",
"background": {
"scripts": ["background.js"]
},
"content_scripts": [{
"matches": ["<all_urls>"],
"js": ["testscript.js"]
}],
"permissions": [
"tabs",
"<all_urls>"
]
}
testscript.js
window.myTest(); // myTest is undefined in the context of the injected script
background.js
// empty, but here to be able to open background page
I also have a pannel that sends a message to the background page when a button is clicked. I know that the panel and sending the message also work. But window.myTest is still undefined.
Edit
Removed the injection from background.js, because I did not use it and have same issue as described.

Answer 1

Finally, I got the specs on this. Mozilla and Chrome follow the same specs for extensions.
Content scripts get a "clean" view of the DOM. This means:
Content scripts cannot see JavaScript variables defined by page
scripts.
If a page script redefines a built-in DOM property, the
content script sees the original version of the property, not the
redefined version.
https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/Content_scripts