Bug 1727289 - Prefer widget scale for scrollbar parts. r=mstange

Print preview documents might use a different DPI depending on the print
target.

Differential Revision: https://phabricator.services.mozilla.com/D123939

widget/nsNativeBasicTheme.cpp

@@ -342,6 +342,9 @@ static std::pair<sRGBColor, sRGBColor> SystemColorPair(
 /* static */
 auto nsNativeBasicTheme::GetDPIRatioForScrollbarPart(nsPresContext* aPc)
     -> DPIRatio {
+  if (auto* widget = aPc->GetRootPresContext()->GetRootWidget()) {
+    return widget->GetDefaultScale();
+  }
   return DPIRatio(float(AppUnitsPerCSSPixel()) /
                   aPc->DeviceContext()->AppUnitsPerDevPixelAtUnitFullZoom());
 }