Bug 1653615 - Ensure displayport snapping doesn't move by more than the margin. r=kats

Differential Revision: https://phabricator.services.mozilla.com/D84552

layout/base/nsLayoutUtils.cpp

@@ -893,6 +893,19 @@ static nsRect GetDisplayPortFromMarginsData(
       float sx = fmin(1.0, (xMargin + w) / w * 0.25);
       posAlignment.width =
           fmax(defaultAlignment, multiple * round(sx * w / multiple));
+      // Margins are usually large (multiple times screenRect's size, see the
+      // apz.x_skate_size_multiplier pref), however it can be be small on
+      // occasions. If the alignment is larger than the margin on the right, we
+      // could end up snapping the displayport to the left too much and miss
+      // some content on the right. Since the size is aligned independently we
+      // can't be sure it will compensate so we ensure that the alignment is
+      // never larger than the margin. The snapping always rounds down so we
+      // don't need to check the left margin.
+      float rightMargin = fabs(aMarginsData->mMargins.right);
+      if (posAlignment.width > rightMargin) {
+        posAlignment.width -=
+            multiple * ceil((posAlignment.width - rightMargin) / multiple);
+      }
     } else {
       posAlignment.width = defaultAlignment;
     }
@@ -902,6 +915,12 @@ static nsRect GetDisplayPortFromMarginsData(
       float sy = fmin(1.0, (yMargin + h) / h * 0.25);
       posAlignment.height =
           fmax(defaultAlignment, multiple * round(sy * h / multiple));
+
+      float bottomMargin = fabs(aMarginsData->mMargins.bottom);
+      if (posAlignment.height > bottomMargin) {
+        posAlignment.height -=
+            multiple * ceil((posAlignment.height - bottomMargin) / multiple);
+      }
     } else {
       posAlignment.height = defaultAlignment;
     }
@@ -988,7 +1007,10 @@ static nsRect GetDisplayPortFromMarginsData(
   ScreenPoint scrollPosScreen =
       LayoutDevicePoint::FromAppUnits(scrollPos, auPerDevPixel) * res;
 
-  // Round-out the display port to the nearest alignment (tiles)
+  // Align the display port.
+  // TODO(bug 1654836): we currently align the origin and independently align
+  // the size. a better approach would be to round out the endpoints of the
+  // unaligned display port.
   screenRect += scrollPosScreen;
   float x = posAlignment.width * floor(screenRect.x / posAlignment.width);
   float y = posAlignment.height * floor(screenRect.y / posAlignment.height);