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tldr; what is the resolution of a screen required so that I can't see pixelation/blurriness of rendered letters, and what hardware is required to achieve this?
my biggest complaint about my current computing setup, debian on a lenovo x230,
is that I can visually see pixelation of the rendering of letters at the edges if I look closely,
especially while running on an external display
one might say this is not a big deal, as the letters are still totally readable, but im a lover of beauty and of crisp beautiful letters, and im curious what would be required to achieve that
@notplants The lcd segmented displays on the old Nintendo Game & Watch have crisp edges 
To actually respond to your question though, the metric you are looking for is "pixel density", similar to "DPI" (dots per inch) from the world of printing.
Celly Phones have really high pixel density screens. You could probably just peek at your friends' phones and see how they look to you, and then look up the model of phone online and see what its pixel density is, then compare that to laptop screens.
Apple is the main hardware vendor for super high pixel density screens on laptops. There are also some other laptops that feature 4k screens, but in general those are the kind of products that I would personally try to avoid as much as possible -- expensive blinged-out and consumer-oriented products that sacrifice functionality for impressive sounding specs.
Cuz here's the thing. The number of pixels goes up as the square of the pixel dimensions of the screen. So the finer the edges of the text get, the exponentially more power the machine will consume, the more complex the software to display things on the screen will have to be, etc.
I think apple actually renders a lot of non-text things at 1/2 scale to give the machines decent performance. When I play video games, even on my 1680x1050 display, I turn down the rendering resolution to make the game smoother. When I'm working with video, especially capturing video, again, I turn down the resolution a lot so that people who I am presenting the video to can actually see WTF I'm doing.
Personally I don't think all the problems that come with using a high resolution display are worth it. If you really just want a 4k display on a laptop tho, I'm sure there are some products out there you could buy, even used ones these days.
@forestjohnson @notplants thanks for this detailed explanation @forestjohnson , I appreciate it, and learned a bunch from reading it.
will have to sit more with whether I actually care about the blurred letters or not, and in general feel you on the weird trade-offs one might have to make to support higher resolution
although on the other hand my macbook of ten years of age really did work well for ten years. also wondering about running linux on old macbooks now, just for the screen… hmmm let’s see