Flat UIs in many cases are objectively bad. The reason is because humans have two eyes, which give us the ability to distinguish depth easily (meaning low cogn

Flat UIs in many cases are objectively bad. The reason is because humans have two eyes, which give us the ability to distinguish depth easily (meaning low cognitive load - our brains are optimized to process this). In a UI, whenever there are more that two dimensions, depth should be shown (simulated, technically) because with our two eyes we cannot discern depth easily on a flat surface. Obviously you can go overboard. For instance, in HN, flat makes sense. But in a windowing system, shadows(1) are essential to show depth. Otherwise it increases the cognitive load required to distinguish between the foreground and background. (1) - you could use blurring (or other techniques) as well, I'm just using shadow as a simple example.