First, try it inside the dev-tools in the browser window:
Before:
After:
So that works. But where is menu.css? Right-click on the name and choose 'Copy link address':
That gave me: chrome-extension://aeblfdkhhhdcdjpifhhbdiojplfjncoa/inline/menu/menu.css.
I use Vivaldi, so I have to look here for that ID 'aeblfdkhhhdcdjpifhhbdiojplfjncoa' : /Users/myusername/Library/Application Support/Vivaldi/Default/Extensions/
You can find the exact folder for your Chrome-based browser, if not Vivaldi, under the URL chrome://version.
I found it and the complete path to 'menu.css' is now: /Users/myusername/Library/Application Support/Vivaldi/Default/Extensions/aeblfdkhhhdcdjpifhhbdiojplfjncoa/8.12.10.36_0/inline/menu/menu.css
Open the file in a text editor and find html{width:300px;max-width:300px} and change that into html{width:max-content;max-width:500px}.
Save and close, reload the page in the browser and voilá.




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