Bug 1059033 - Part 6: Clip text drawing to its bound so we can skip unaffected tiles. r=Bas

layout/generic/nsTextFrame.cpp

@@ -4565,8 +4565,18 @@ nsDisplayText::Paint(nsDisplayListBuilder* aBuilder,
   extraVisible.Inflate(appUnitsPerDevPixel, appUnitsPerDevPixel);
   nsTextFrame* f = static_cast<nsTextFrame*>(mFrame);
 
-  gfxContextAutoDisableSubpixelAntialiasing disable(aCtx->ThebesContext(),
+  gfxContext* ctx = aCtx->ThebesContext();
+  gfxContextAutoDisableSubpixelAntialiasing disable(ctx,
                                                     mDisableSubpixelAA);
+  gfxContextAutoSaveRestore save(ctx);
+
+  gfxRect pixelVisible =
+    nsLayoutUtils::RectToGfxRect(extraVisible, appUnitsPerDevPixel);
+
+  ctx->NewPath();
+  ctx->Rectangle(pixelVisible);
+  ctx->Clip();
+
   NS_ASSERTION(mLeftEdge >= 0, "illegal left edge");
   NS_ASSERTION(mRightEdge >= 0, "illegal right edge");
   f->PaintText(aCtx, ToReferenceFrame(), extraVisible, *this);