Bug 1656802. Add a comment explaining how overlay scrollbars work with their negative margin. r=emilio

Differential Revision: https://phabricator.services.mozilla.com/D85703

layout/generic/nsGfxScrollFrame.cpp

@@ -6593,6 +6593,10 @@ void ScrollFrameHelper::LayoutScrollbars(nsBoxLayoutState& aState,
                               : mScrollPort.x + compositionSize.width;
     if (mHasVerticalScrollbar) {
       nsMargin margin;
+      // For overlay scrollbars the margin returned from this GetXULMargin call
+      // is a negative margin that moves the scrollbar from just outside the
+      // scrollport (and hence not visible) to just inside the scrollport (and
+      // hence visible). For non-overlay scrollbars it is a 0 margin.
       mVScrollbarBox->GetXULMargin(margin);
       vRect.Deflate(margin);
     }
@@ -6611,6 +6615,10 @@ void ScrollFrameHelper::LayoutScrollbars(nsBoxLayoutState& aState,
     hRect.y = mScrollPort.y + compositionSize.height;
     if (mHasHorizontalScrollbar) {
       nsMargin margin;
+      // For overlay scrollbars the margin returned from this GetXULMargin call
+      // is a negative margin that moves the scrollbar from just outside the
+      // scrollport (and hence not visible) to just inside the scrollport (and
+      // hence visible). For non-overlay scrollbars it is a 0 margin.
       mHScrollbarBox->GetXULMargin(margin);
       hRect.Deflate(margin);
     }