U3219Q, dual systems PC and Mac, split screen, keyboard mouse?

@Wil-l  If you want your keyboard and mouse to be shared by the display, you'd need those devices or their receivers plugged into the display AND you'll need a USB data path from the display back to each system.  Connecting your Precision via DisplayPort only creates a video output link, not a USB data path.  If you want that, you'll need to connect a USB 3.0 cable from the display's USB upstream port to the Precision as well.  The USB-C connection to the MacBook Pro will carry both video and USB along the same cable, although depending on the model of that system, you might have to choose between 4K 60 Hz and only USB 2.0, or 4K 30 Hz and USB 3.0.  You might not be able to run 4K 60 Hz and USB 3.0 simultaneously along the same cable.  (Fyi when asking technical questions it helps to provide basic technical info, like the exact models of all equipment involved.)

But note that even in that setup with your peripherals running through the display, the display won't dynamically switch the peripherals between systems like Logitech Flow.  The display has no way of knowing where the cursor is and therefore which system should receive the signals coming from your keyboard and mouse.  The way the KVM function built into that display works is that you assign the USB 3.0 upstream port to a specific video input so that when that video input is active, the USB data path through that upstream port is also active rather than the USB-C data path.  And when in PBP/split screen, you have to choose which ONE of your USB data paths to be active.

And lastly, if you plan to use PBP on a regular basis, a 16:9 display really isn't a great choice because that doesn't split up well either horizontally or vertically.  You either end up with two tiles that are each taller than they are wide (8:9 aspect ratio), i.e. the opposite of wide screen, or else you end up with two tiles that are very wide but very squat.  If you want to go full-time split screen, a better display choice might be the U4919DW, which is a 5120x1440 32:9 aspect ratio display.  When THAT is used split screen, each side runs at QHD (2560x1440), which is a standard 16:9 aspect ratio and the most common resolution for 27" displays.