So gl_FragCoord.xy goes from 0 to screen/window width and height.
And uniform vec2 uResolution passes stationary window width and height.

Assuming the window is full screen on my 1920 * 1080 display,

vec2 uv = gl_FragCoord.xy / uResolution.xy;

gl_FragColor = vec4( uv, .0, 1.0);

should result in a normalized uv coordinate system with bottom left being 0, 0 (i.e. 0/1920, 0/1080) and top right being 1, 1 (i.e. 1920/1920, 1080/1080)

But why does the top range on both axis get severely overshot and shrinks the 0-1 range in the bottom left like the first image?

Because applying a simple fract to this uv, uv = fract( uv );, gives the divisions, as in the 2nd image, like if I multiplied uv by 5.: