Bug 1204932 - When C++APZ is enabled, elements that are zoomed with double tap should be centered. r=botond

gfx/layers/apz/src/AsyncPanZoomController.cpp

@@ -3130,13 +3130,16 @@ void AsyncPanZoomController::ZoomToRect(CSSRect aRect) {
       targetZoom = CSSToParentLayerScale(std::min(compositionBounds.width / aRect.width,
                                                   compositionBounds.height / aRect.height));
     }
-    // 1. If the rect is empty, request received from browserElementScrolling.js
+    // 1. If the rect is empty, the content-side logic for handling a double-tap
+    //    requested that we zoom out.
     // 2. currentZoom is equal to mZoomConstraints.mMaxZoom and user still double-tapping it
     // 3. currentZoom is equal to localMinZoom and user still double-tapping it
     // Treat these three cases as a request to zoom out as much as possible.
+    bool zoomOut = false;
     if (aRect.IsEmpty() ||
         (currentZoom == localMaxZoom && targetZoom >= localMaxZoom) ||
         (currentZoom == localMinZoom && targetZoom <= localMinZoom)) {
+      zoomOut = true;
       CSSSize compositedSize = mFrameMetrics.CalculateCompositedSizeInCssPixels();
       float y = scrollOffset.y;
       float newHeight =
@@ -3170,6 +3173,14 @@ void AsyncPanZoomController::ZoomToRect(CSSRect aRect) {
       aRect.x = aRect.x > 0 ? aRect.x : 0;
     }
 
+    // Vertically center the zoomed element in the screen.
+    if (!zoomOut && (sizeAfterZoom.height > aRect.height)) {
+      aRect.y -= (sizeAfterZoom.height - aRect.height) * 0.5f;
+      if (aRect.y < 0.0f) {
+        aRect.y = 0.0f;
+      }
+    }
+
     endZoomToMetrics.SetScrollOffset(aRect.TopLeft());
     endZoomToMetrics.SetDisplayPortMargins(
       CalculatePendingDisplayPort(endZoomToMetrics,