Ok, so this Dooy "fauxdini" sun jacket is an interesting garment. Linky above.
It is feather light and gossamer thin. Some have weighed them around 70g. It is slightly translucent when there's some back light, but not so much that it looks strange wearing it. It packs down into a fist-sized bag or the chest pocket and it is still squishy, so it will fit in a shorts pocket. You could probably stuff it into a bag half the size and it would be pretty taut.
It is actually water repellent, about like a "regular" windbreaker (soaks through after a while and any seams let moisture through).
It breathes about like a face mask, so definitely breathable but would seem to retain some wind cutting ability. It's actually pretty comfortable, even against bare skin as the slight texture to the fabric and breathability keep it from being instantly cold and clammy like a plain nylon jacket can be.
Fit is decent. I got an XL and I'm 6-1 about 180 and it's fine, sleeves maybe slightly short relative to body, but still long enough for my 34.5 sleeve length. It is a trim fit, like many of these are, so may not go over bulky fleeces very well. Build obviously isn't Patagonia quality, but it's decent. Seams aren't taped, but they are "lapped" or "felled," so not just open stitching. Raglan sleeves, which are nice.
Like a lot of ultralight gear, it feels flimsy, but that is probably a bit deceptive. It's not going to withstand many encounters with greenbriar or other bushes, but how many of this type really are.
It should be mid-50s today, so I may be able to test it out over a long-sleeve poly base layer, which is where I'm thinking this type of garment excels. Or, I may get out earlier, in the 40s and try it over a fleece.