The Steam client on Windows, macOS, and Linux has been updated with support for new features pertinent to each platform, as well as with a number of general bugfixes applicable across all of them.
Broader fixes include reduced CPU usage during interface redraws, such as rolling your mouse over objects in the library view, and improved algorithms that attempt to avoid downloads as much as possible when importing games from retail discs or backup files.
Windows-specific features include high-DPI display support for all PCs running Windows 10 Fall Creators Update, better taskbar behavior for multi-monitor systems, support for exFAT-formatted drives, and animated transitions for the maximizing/minimizing/closing of Steam windows.
On the macOS side of things, you can now turn off or reboot the system from Steam's Big Picture mode. In addition, the Steam client's windows now come with native integration with macOS, which would allow you to drag it past the menu bar up top and trigger the Spaces UI.
Among other features, the Linux variant of the Steam client now has a 2x-scaling mode for better compatibility with 4K monitors. Similar to macOS, the client should also now play better with Linux' windows manager while resizing it.
There are a couple of relatively minor fixes that weren't mentioned above, along with a massive host of patches for Steam Input, all of which you can check out over here.