diff --git a/tools/sk_app/VulkanWindowContext.cpp b/tools/sk_app/VulkanWindowContext.cpp
index 66670c892e..3a6804166f 100644
--- a/tools/sk_app/VulkanWindowContext.cpp
+++ b/tools/sk_app/VulkanWindowContext.cpp
@@ -553,7 +553,7 @@ sk_sp<SkSurface> VulkanWindowContext::getBackbufferSurface() {
     return sk_ref_sp(surface);
 }
 
-void VulkanWindowContext::swapBuffers() {
+void VulkanWindowContext::swapBuffers(const SkIRect*) {
 
     BackbufferInfo* backbuffer = fBackbuffers + fCurrentBackbufferIndex;
     SkSurface* surface = fSurfaces[backbuffer->fImageIndex].get();
diff --git a/tools/sk_app/VulkanWindowContext.h b/tools/sk_app/VulkanWindowContext.h
index 07a18a46a9..aa6f95e2a1 100644
--- a/tools/sk_app/VulkanWindowContext.h
+++ b/tools/sk_app/VulkanWindowContext.h
@@ -48,7 +48,7 @@ public:
     static SharedGrDirectContext getSharedGrDirectContext() { return SharedGrDirectContext( fGlobalShared ); }
 
     sk_sp<SkSurface> getBackbufferSurface() override;
-    void swapBuffers() override;
+    void swapBuffers(const SkIRect* rect = nullptr) override;
 
     bool isValid() override { return fSurface != VK_NULL_HANDLE; }
 
diff --git a/tools/sk_app/WindowContext.h b/tools/sk_app/WindowContext.h
index 6920e5ba89..219330a61b 100644
--- a/tools/sk_app/WindowContext.h
+++ b/tools/sk_app/WindowContext.h
@@ -8,6 +8,7 @@
 #define WindowContext_DEFINED
 
 #include "include/core/SkRefCnt.h"
+#include "include/core/SkRect.h"
 #include "include/core/SkSurfaceProps.h"
 #include "include/gpu/GrTypes.h"
 #include "include/gpu/GrDirectContext.h"
@@ -29,7 +30,7 @@ public:
 
     virtual sk_sp<SkSurface> getBackbufferSurface() = 0;
 
-    virtual void swapBuffers() = 0;
+    virtual void swapBuffers(const SkIRect* rect = nullptr) = 0;
 
     virtual bool isValid() = 0;
 
diff --git a/tools/sk_app/unix/RasterWindowContext_unix.cpp b/tools/sk_app/unix/RasterWindowContext_unix.cpp
index e8ae942308..fc06b40069 100644
--- a/tools/sk_app/unix/RasterWindowContext_unix.cpp
+++ b/tools/sk_app/unix/RasterWindowContext_unix.cpp
@@ -19,7 +19,7 @@ public:
     RasterWindowContext_xlib(Display*, XWindow, int width, int height, const DisplayParams&);
 
     sk_sp<SkSurface> getBackbufferSurface() override;
-    void swapBuffers() override;
+    void swapBuffers(const SkIRect* rect) override;
     bool isValid() override { return SkToBool(fWindow); }
     void resize(int  w, int h) override;
     void setDisplayParams(const DisplayParams& params) override;
@@ -60,7 +60,7 @@ void RasterWindowContext_xlib::resize(int  w, int h) {
 
 sk_sp<SkSurface> RasterWindowContext_xlib::getBackbufferSurface() { return fBackbufferSurface; }
 
-void RasterWindowContext_xlib::swapBuffers() {
+void RasterWindowContext_xlib::swapBuffers(const SkIRect* rect) {
     SkPixmap pm;
     if (!fBackbufferSurface->peekPixels(&pm)) {
         return;
@@ -82,7 +82,9 @@ void RasterWindowContext_xlib::swapBuffers() {
     if (!XInitImage(&image)) {
         return;
     }
-    XPutImage(fDisplay, fWindow, fGC, &image, 0, 0, 0, 0, pm.width(), pm.height());
+    SkIRect update = rect ? *rect : SkIRect::MakeWH( pm.width(), pm.height());
+    XPutImage(fDisplay, fWindow, fGC, &image, update.x(), update.y(),
+              update.x(), update.y(), update.width(), update.height());
 }
 
 }  // anonymous namespace
diff --git a/tools/sk_app/win/RasterWindowContext_win.cpp b/tools/sk_app/win/RasterWindowContext_win.cpp
index 49f1f9ed17..f0db1f6f06 100644
--- a/tools/sk_app/win/RasterWindowContext_win.cpp
+++ b/tools/sk_app/win/RasterWindowContext_win.cpp
@@ -22,7 +22,7 @@ public:
     RasterWindowContext_win(HWND, const DisplayParams&);
 
     sk_sp<SkSurface> getBackbufferSurface() override;
-    void swapBuffers() override;
+    void swapBuffers(const SkIRect* rect) override;
     bool isValid() override { return SkToBool(fWnd); }
     void resize(int w, int h) override;
     void setDisplayParams(const DisplayParams& params) override;
@@ -75,13 +75,17 @@ void RasterWindowContext_win::resize(int w, int h) {
 
 sk_sp<SkSurface> RasterWindowContext_win::getBackbufferSurface() { return fBackbufferSurface; }
 
-void RasterWindowContext_win::swapBuffers() {
+void RasterWindowContext_win::swapBuffers(const SkIRect* rect) {
     BITMAPINFO* bmpInfo = reinterpret_cast<BITMAPINFO*>(fSurfaceMemory.get());
     HDC dc = GetDC(fWnd);
     SkPixmap pixmap;
     fBackbufferSurface->peekPixels(&pixmap);
-    StretchDIBits(dc, 0, 0, fWidth, fHeight, 0, 0, fWidth, fHeight, pixmap.addr(), bmpInfo,
-                  DIB_RGB_COLORS, SRCCOPY);
+    SkIRect update = rect ? *rect : SkIRect::MakeWH( fWidth, fHeight );
+    // It appears that y-axis handling is broken if it doesn't match the window size.
+    update = SkIRect::MakeXYWH( update.x(), 0, update.width(), fHeight );
+    StretchDIBits(dc, update.x(), update.y(), update.width(), update.height(),
+                  update.x(), update.y(), update.width(), update.height(),
+                  pixmap.addr(), bmpInfo, DIB_RGB_COLORS, SRCCOPY);
     ReleaseDC(fWnd, dc);
 }