Fix wxMemoryDC::Blit() with itself as source in wxMSW

Don't use StretchDIBits() native function as it doesn't seem to handle the
case when its source and destination are the same correctly.

Closes #17666.

docs/changes.txt

@@ -134,6 +134,7 @@ wxMSW:
 - Fix wxGraphicsMatrix::TransformDistance for Direct2D renderer.
 - Fix wxDC::Clear() for rotated DC.
 - Fix wxDC::GetClippingBox() for transformed wxDC.
+- Fix wxMemoryDC::Blit() with itself as source (Tim Roberts).
 
 wxOSX:
 

src/msw/dc.cpp

@@ -2294,8 +2294,10 @@ bool wxMSWDCImpl::DoStretchBlit(wxCoord xdest, wxCoord ydest,
         // if we already have a DIB, draw it using StretchDIBits(), otherwise
         // use StretchBlt() if available and finally fall back to BitBlt()
 
+        // Notice that we can't use StretchDIBits() when source and destination
+        // are the same as it can't, apparently, handle overlapping surfaces.
         const int caps = ::GetDeviceCaps(GetHdc(), RASTERCAPS);
-        if ( bmpSrc.IsOk() && (caps & RC_STRETCHDIB) )
+        if ( bmpSrc.IsOk() && (GetHdc() != hdcSrc) && (caps & RC_STRETCHDIB) )
         {
             DIBSECTION ds;
             wxZeroMemory(ds);

tests/graphics/bitmap.cpp

@@ -17,8 +17,14 @@
 #endif
 
 #include "wx/bitmap.h"
+#include "wx/rawbmp.h"
 #include "wx/dcmemory.h"
 
+#define ASSERT_EQUAL_RGB(c, r, g, b) \
+    CPPUNIT_ASSERT_EQUAL( r, (int)c.Red() ); \
+    CPPUNIT_ASSERT_EQUAL( g, (int)c.Green() ); \
+    CPPUNIT_ASSERT_EQUAL( b, (int)c.Blue() )
+
 // ----------------------------------------------------------------------------
 // test class
 // ----------------------------------------------------------------------------
@@ -34,9 +40,11 @@ public:
 private:
     CPPUNIT_TEST_SUITE( BitmapTestCase );
         CPPUNIT_TEST( Mask );
+        CPPUNIT_TEST( OverlappingBlit );
     CPPUNIT_TEST_SUITE_END();
 
     void Mask();
+    void OverlappingBlit();
 
     wxBitmap m_bmp;
 
@@ -80,3 +88,35 @@ void BitmapTestCase::Mask()
     m_bmp.SetMask(mask2);
 }
 
+void BitmapTestCase::OverlappingBlit()
+{
+    m_bmp.SetMask( NULL );
+
+    // Clear to white.
+
+    wxMemoryDC dc(m_bmp);
+    dc.SetBackground( *wxWHITE );
+    dc.Clear();
+
+    // Draw red line across the top.
+
+    dc.SetPen(*wxRED_PEN);
+    dc.DrawLine(0, 0, 10, 0);
+
+    // Scroll down one line.
+
+    dc.Blit( 0, 1, 10, 9, &dc, 0, 0 );
+
+    // Now, lines 0 and 1 should be red, lines 2++ should still be white.
+
+    wxNativePixelData npd( m_bmp );
+    wxNativePixelData::Iterator it( npd );
+
+    ASSERT_EQUAL_RGB( it, 255, 0, 0 );
+    it.OffsetY( npd, 1 );
+    ASSERT_EQUAL_RGB( it, 255, 0, 0 );
+    it.OffsetY( npd, 1 );
+    ASSERT_EQUAL_RGB( it, 255, 255, 255 );
+    it.OffsetY( npd, 1 );
+    ASSERT_EQUAL_RGB( it, 255, 255, 255 );
+}