fix: Use correct window to get the devicePixelRatio

The `roundOffsetsByDPR` utility was using the global window keyword
instead of the `getWindow` utility. This meant that applications that
render to childe windows would have blurry poppers sincee the device pixel
ratio being used wouldn't be correct.Two cases where that could
happen:

* Parent window in another monitor
* Parent window has a different zoom than the child window

src/modifiers/computeStyles.js

@@ -46,8 +46,7 @@ const unsetSides = {
 // Round the offsets to the nearest suitable subpixel based on the DPR.
 // Zooming can change the DPR, but it seems to report a value that will
 // cleanly divide the values into the appropriate subpixels.
-function roundOffsetsByDPR({ x, y }): Offsets {
-  const win: Window = window;
+function roundOffsetsByDPR({ x, y }, win: Window): Offsets {
   const dpr = win.devicePixelRatio || 1;
 
   return {
@@ -150,7 +149,7 @@ export function mapToStyles({
 
   ({ x, y } =
     roundOffsets === true
-      ? roundOffsetsByDPR({ x, y })
+      ? roundOffsetsByDPR({ x, y }, getWindow(popper))
       : { x, y });
 
   if (gpuAcceleration) {