No way to determine native DPR of device

[ebidel]

The following code doesn't allow someone to use the device's native DPR by setting await page.setViewport({deviceScaleFactor: 0,...}):

https://github.com/GoogleChrome/puppeteer/blob/master/lib/Page.js#L729
https://github.com/GoogleChrome/puppeteer/blob/294f33b75c5a7c87d447816218438f56c2d56f0e/lib/EmulationManager.js

That makes it impossible to determine the real DPR of the device:

await page.evaluate('window.devicePixelRatio'); 

[JoelEinbinder]

Easy fix, but why do you need the native DPR? It should be largely irrelevant in headless afaik.

[ebidel]

You want to know this information for taking high quality screenshots.

For my particular case, I'm running an app in both mac and linux. So I want to use the correct DPR per the device without hard coding numbers.

[JoelEinbinder]

Why would the dpr of the device affect the screenshot quality? The device doesn’t necessarily have a single dpr

[ebidel]

You need to emulate a viewport with deviceScaleFactor: 2 to produce crisp screenshots on a retina mac. On Linux, using that produces a screenshot that's 2x what you actually need...and it's buggy. See our pptr chat.

[aslushnikov]

Let's introduce a viewport option to the launcher as a part of #1183. One of the values could be null to disable viewport override altogether.