Bug 721070 - Checkerboarding at bottom after pinch zooming out. r=kats,pcwalton

Certain conditions would cause MultiTileLayer to incorrectly invalidate tiles,
or to reuse tiles incorrectly, which would cause checkerboarding (sometimes
permanent until a redraw occurs).

This addresses said issues:

- Removes manual invalidation on Document:Shown and Tab:Selected. This is
  unnecessary, as the entire buffer will be invalidated when this happens
  anyway. Sometimes Document:Shown happens *after* drawing has occurred,
  which caused the entire screen to checkerboard incorrectly.
- Fix re-using off-screen tiles with the incorrect resolution. Tiles are stored
  in a hash-map based on their unscaled position. As only tiles that intersect
  with the update region in tile-space are invalidated, sometimes tiles that
  were off-screen, but whose unscaled tile origin appears on-screen would be
  incorrectly re-used. Fix this by checking that a tile's resolution matches the
  current resolution before reusing it.

mobile/android/base/gfx/GeckoSoftwareLayerClient.java

@@ -162,8 +162,6 @@ public class GeckoSoftwareLayerClient extends LayerClient implements GeckoEventL
 
         GeckoAppShell.registerGeckoEventListener("Viewport:UpdateAndDraw", this);
         GeckoAppShell.registerGeckoEventListener("Viewport:UpdateLater", this);
-        GeckoAppShell.registerGeckoEventListener("Document:Shown", this);
-        GeckoAppShell.registerGeckoEventListener("Tab:Selected:Done", this);
 
         sendResizeEventIfNecessary();
     }
@@ -252,7 +250,6 @@ public class GeckoSoftwareLayerClient extends LayerClient implements GeckoEventL
             Point tileOrigin = new Point((origin.x / TILE_SIZE.width) * TILE_SIZE.width,
                                          (origin.y / TILE_SIZE.height) * TILE_SIZE.height);
             mRenderOffset.set(origin.x - tileOrigin.x, origin.y - tileOrigin.y);
-            ((MultiTileLayer)mTileLayer).invalidateBuffer();
         }
 
         // If the window size has changed, reallocate the buffer to match.
@@ -519,16 +516,6 @@ public class GeckoSoftwareLayerClient extends LayerClient implements GeckoEventL
             GeckoAppShell.sendEventToGecko(new GeckoEvent(GeckoEvent.DRAW, rect));
         } else if ("Viewport:UpdateLater".equals(event)) {
             mUpdateViewportOnEndDraw = true;
-        } else if (("Document:Shown".equals(event) ||
-                    "Tab:Selected:Done".equals(event)) &&
-                   (mTileLayer instanceof MultiTileLayer)) {
-            beginTransaction(mTileLayer);
-            try {
-                ((MultiTileLayer)mTileLayer).invalidateTiles();
-                ((MultiTileLayer)mTileLayer).invalidateBuffer();
-            } finally {
-                endTransaction(mTileLayer);
-            }
         }
     }
 

mobile/android/base/gfx/MultiTileLayer.java

@@ -333,7 +333,7 @@ public class MultiTileLayer extends Layer {
                 }
 
                 // Dirty tile, find out if we already have this tile to reuse.
-                boolean reusedTile = true;
+                boolean reusedTile;
                 Point tileOrigin = new Point(x, y);
                 SubTile tile = mPositionHash.get(longFromPoint(tileOrigin));
 
@@ -343,6 +343,10 @@ public class MultiTileLayer extends Layer {
                     reusedTile = false;
                 } else {
                     mTiles.remove(tile);
+
+                    // Reuse the tile (i.e. keep the texture data and metrics)
+                    // only if the resolution matches
+                    reusedTile = FloatUtils.fuzzyEquals(tile.getResolution(), getResolution());
                 }
 
                 // Place tile at the end of the tile-list so it isn't re-used.
@@ -396,10 +400,36 @@ public class MultiTileLayer extends Layer {
     @Override
     public void draw(RenderContext context) {
         for (SubTile layer : mTiles) {
+            // Skip invalid tiles
+            if (layer.key == null) {
+                continue;
+            }
+
             // Avoid work, only draw tiles that intersect with the viewport
             RectF layerBounds = layer.getBounds(context, new FloatSize(layer.getSize()));
-            if (RectF.intersects(layerBounds, context.viewport))
+            if (RectF.intersects(layerBounds, context.viewport)) {
                 layer.draw(context);
+            }
+        }
+    }
+
+    @Override
+    public void setOrigin(Point origin) {
+        Point oldOrigin = getOrigin();
+
+        if (!origin.equals(oldOrigin)) {
+            super.setOrigin(origin);
+            invalidateBuffer();
+        }
+    }
+
+    @Override
+    public void setResolution(float resolution) {
+        float oldResolution = getResolution();
+
+        if (!FloatUtils.fuzzyEquals(resolution, oldResolution)) {
+            super.setResolution(resolution);
+            invalidateBuffer();
         }
     }