exampleComponentReworkedWithReactive.vue

<template lang="pug">
  div.component(ref="el" :style="{ filter: `saturate(${saturation}) brightness(${brightness})` }")
    div
      div Brightness: {{brightness}}
      div Saturation: {{saturation}}
      div Width: {{width}}
      div Height: {{height}}
      div X: {{x}}
      div Y: {{y}}
</template>

<script>
import { onBeforeUnmount, onMounted, reactive, ref, toRefs, watch } from '@vue/composition-api'
export default {
  setup () {
    let el = ref(null)

    const { x, y } = useRelativeMousePosition(el)
    const { width, height } = useSizeObserver(el)
    const { brightness } = useBrightness(x, width)
    const { saturation } = useSaturation(y, height)

    return {
      el,
      width,
      height,
      x,
      y,
      brightness,
      saturation
    }
  }
}

function useRelativeMousePosition (el) {
  let mousePosition = reactive({
    x: 0,
    y: 0
  })

  function update (event) {
    const rect = el.getBoundingClientRect()
    mousePosition.x = event.clientX - rect.left
    mousePosition.y = event.clientY - rect.top
  }

  onMounted(() => window.addEventListener('mousemove', update))
  onBeforeUnmount(() => window.removeEventListener('mousemove', update))

  return toRefs(mousePosition)

function useSizeObserver (el) {
  let attrs = reactive({
    width: 0,
    height: 0
  })

  const resizeObserver = new ResizeObserver(entries => {
    for (let entry of entries) {
      attrs.width = entry.contentRect.width
      attrs.height = entry.contentRect.height
    }
  })
  onMounted(() => resizeObserver.observe(el))
  onBeforeUnmount(() => resizeObserver.unobserve(el))
  return toRefs(attrs)
}

function useBrightness (value, maxValue) {
  let brightnessRef = ref(0)
  watch([value, maxValue], ([value, max]) => {
    brightnessRef = cappedValue(value, { max, min: 0 })
  }, { lazy: true })
  return { brightnessRef }
}

function useSaturation (value, maxValue) {
  let saturationRef = ref(0)
  watch([value, maxValue], ([value, max]) => {
    saturationRef = cappedValue(value, { max, min: 0 })
  }, { lazy: true })
  return { saturationRef }
}

function cappedValue (value, { max, min }) {
  return Math.max(min, Math.min(value, max)) / max
}
</script>

<style
  lang="scss"
  scoped
>
  .component {
    max-width: 700px;
    min-height: 300px;
    background: royalblue;
    color: white;
    display: flex;
    align-items: center;
    justify-content: center;
    flex-direction: column;
  }
</style>

@trafium - edited. Will that work?

Scott