static void
RecenterDisplayPort(mozilla::layers::FrameMetrics& aFrameMetrics)
{
LayerMargin margins = aFrameMetrics.GetDisplayPortMargins();
margins.right = margins.left = margins.LeftRight() / 2;
margins.top = margins.bottom = margins.TopBottom() / 2;
aFrameMetrics.SetDisplayPortMargins(margins);
}
static void
AdjustDisplayPortForScrollDelta(mozilla::layers::FrameMetrics& aFrameMetrics,
const CSSPoint& aActualScrollOffset)
{
// Correct the display-port by the difference between the requested scroll
// offset and the resulting scroll offset after setting the requested value.
LayerPoint shift =
(aFrameMetrics.GetScrollOffset() - aActualScrollOffset) *
aFrameMetrics.LayersPixelsPerCSSPixel();
LayerMargin margins = aFrameMetrics.GetDisplayPortMargins();
margins.left -= shift.x;
margins.right += shift.x;
margins.top -= shift.y;
margins.bottom += shift.y;
aFrameMetrics.SetDisplayPortMargins(margins);
}