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I've got an extension: https://addons.mozilla.org/en-US/firefox/addon/bookmark-dial/ (and there are several others) that generate thumbnails. I know there is a service today that does it, but it won't be accessible from WebExtensions. It would be great if there would be a WebExtension API that I can call with a URL and that generates a thumbnail and returns a URL to it. Ideally there would be a way to modify the CSS for it (this is how I do it now: https://github.com/sblask/firefox-bookmark-dial/blob/master/lib/thumbnail.js#L89 / https://github.com/sblask/firefox-bookmark-dial/blob/master/data/thumbnail_tab_content_script.js) Alternatively, there could be streamlined version of what I do today: open a tab, hide it, apply style and capture it. What would be missing is hiding a tab (this is what I do today: https://github.com/sblask/firefox-bookmark-dial/blob/master/lib/thumbnail.js#L30 (collapsed + disabled)) and capturing any tab(not just the currently visible one) to allow for background generation.
Is this covered by chrome.tabs.captureVisibleTab() ? https://developer.mozilla.org/en-US/Add-ons/WebExtensions/API/Tabs/captureVisibleTab
No, because any tab should be capturable, not just the currently visible one.
This seems like a good idea, we haven't talked about it much because the whiteboard tag was wrong, but I can't imagine this being that controversial and I don't think there's anything that does this already. As a minor point I'm commenting on this bug using VerticalTabs which is showing a thumbnail of the window in the tab...
looking at this sample-code from developer.chrome.com there is an api in chrome to get the thumbnail.  "permissions": [ "topSites", "chrome://favicon/" ], --  eightBallWindow.style.backgroundImage = 'url(chrome://favicon/' + data[rand].url + ')';  https://developer.chrome.com/extensions/examples/api/topsites/magic8ball/manifest.json  https://developer.chrome.com/extensions/examples/api/topsites/magic8ball/newTab.js