We did some more tests here and all seems to work fine.
Example:
Kantu’s Resize | 1024@768 command brings the browser in exactly the same dimensions as if you would use e. g. the Window Resizer tool. Note that you need to select “viewport” to compare.
But you are also correct : If you take a screenshot of the browser on a HiDPI machine (like your laptop) the dimensions will be scaled. For example, I am currently connected to a 120dpi monitor, so the 1024@768 resized browser gives a screenshot of 1260x962px (factor 120dpi/96dpi).
I record gif animation from screen. As you can see first attempt is not a 375x667, its near 915x800
And when i put resize in target, screen resized to proper size.
I upload screencast on disk.yandex.ru because error when upload this gif on forum: