Windows 10 Settings includes an option:
When i snap a window, automatically size it to fill available space
System - Multitasking)
I don't see any change between when it's enabled or disabled. What does it do?
UPDATE: More learned in comments.
The specific setting
When i snap a window, automatically size it to fill available space controls the behavior of a window that is snapping into a space where there are already other snapped windows.
If this setting is enabled, when you have snapped one window into an available space and then resized that window, snapping a new window will fill all of the space left over.
If this setting is disabled, when you have snapped and resized one window, subsequent windows will snap to their default half or quarter size and will either overlap existing snapped windows, or "underlap" and leave a gap.
You can test and observe this behavior using the following steps:
Windows 10 supports the following snap actions with windows:
Note: When dragging the window, the snap action does not initiate until the mouse cursor that is dragging the window enters the interaction zone. So, if you click in roughly the middle of the window's title bar and drag it off to the right edge of your screen, it will not Snap until half the window is off of the edge of the screen and your mouse cursor actually hits the edge of the screen.
There is also a visual cue regarding what will happen if you release the mouse in a given location in the form of a shadow box showing the shape and size the window will Snap to if it is released there.
Try dragging a window around the periphery of your screen and observe how this shadow box changes shape depending on whether your mouse cursor is in a corner, on the left or right edge of the screen, or against the top of the screen.
I've found this to be a rather handy feature, personally. Especially if you're used to maximizing your windows, this can make it quite easy to get used to having more than one program up front and visible on your screen at the same time.
It is also based on touch gestures. If you have a touch screen, you can perform the same window arrangement using your finger to drag the program window up against the top or sides or corners to arrange them.