Noctua released a new Mini-ITX cooler, the NH-L9x65. At 51mm tall (without fan), the heat sink is by far the tallest I can find that fits comfortably in the Lian Li PC-TU100 which has a cooler height limit of 60mm. The idea is to rely on the PSU fan to pull hot air from the CPU heat sink as opposed to a dedicated CPU fan, so that I can reduce the number of fans in the system and thereby reduce noise.
Previous performance Mini-ITX coolers have always been wide (and thick), which means risks of interference with memory and PCIe devices. Even when compatibility is guaranteed, a fat cooler can make installation challenging because it might block access to power connectors, SATA ports, front-panel jumpers, etc. Using a fat cooler often mandates a particular order of installation of components; and often you cannot modify one component without taking the whole thing apart. For these reasons I have always wanted a cooler that keeps its lateral footprint within the socket keep-out zone (95mm by 95mm) and takes more advantage of the vertical space above the socket. Examples of coolers that do the former but not the latter include the Noctua NH-L9i/L9a and Phanteks PH-TC90LS. Coolers that do the latter mostly violate the former, such as the Thermalright AXP-200, Noctua NH-L12, and SilverStone NT06-PRO. The Noctua NH-L9x65 fills the niche nicely. It will not be a performance cooler in absolute terms, but it is one that offers maximum performance while retaining maximum compatibility.