Fixed setting clipping box.

While setting a clipping box there is necessary to intersect it either with current clipping region location if such region exists or with wxDC surface extents if no clipping region is set. This way effective clipping box will be always inside the wxDC surface.
Note: Effective clipping box can be an empty region.

See #17013

interface/wx/dc.h

@@ -767,13 +767,15 @@ public:
         uses for the clipping region are for clipping text or for speeding up
         window redraws when only a known area of the screen is damaged.
 
-        Notice that you need to call DestroyClippingRegion() if you want to set
+        @remarks
+        - Calling GetClippingBox() can only make the clipping region smaller,
+        never larger.
+
+        - You need to call DestroyClippingRegion() if you want to set
         the clipping region exactly to the region specified.
 
-        Also note that if the clipping region is empty, any previously set
-        clipping region is destroyed, i.e. it is equivalent to calling
-        DestroyClippingRegion(), and not to clipping out all drawing on the DC
-        as might be expected.
+        - If resulting clipping region is empty, then all drawing on the DC is
+        clipped out (all changes made by drawing operations are masked out).
 
         @see DestroyClippingRegion(), wxRegion
     */

src/common/dcbase.cpp

@@ -368,21 +368,41 @@ wxDCImpl::~wxDCImpl()
 
 void wxDCImpl::DoSetClippingRegion(wxCoord x, wxCoord y, wxCoord w, wxCoord h)
 {
+    wxASSERT_MSG( w >= 0 && h >= 0,
+                  wxS("Clipping box size values cannot be negative") );
+
+    wxRect newRegion(x, y, w, h);
+
+    wxRect clipRegion;
     if ( m_clipping )
     {
-        m_clipX1 = wxMax( m_clipX1, x );
-        m_clipY1 = wxMax( m_clipY1, y );
-        m_clipX2 = wxMin( m_clipX2, (x + w) );
-        m_clipY2 = wxMin( m_clipY2, (y + h) );
+        // New clipping box is an intersection
+        // of required clipping box and the current one.
+        wxRect curRegion(m_clipX1, m_clipY1, m_clipX2 - m_clipX1, m_clipY2 - m_clipY1);
+        clipRegion = curRegion.Intersect(newRegion);
     }
     else
     {
-        m_clipping = true;
+        // Effective clipping box is an intersection
+        // of required clipping box and DC surface.
+        int dcWidth, dcHeight;
+        DoGetSize(&dcWidth, &dcHeight);
+        wxRect dcRect(wxSize(dcWidth, dcHeight));
+        clipRegion = dcRect.Intersect(newRegion);
 
-        m_clipX1 = x;
-        m_clipY1 = y;
-        m_clipX2 = x + w;
-        m_clipY2 = y + h;
+        m_clipping = true;
+    }
+
+    if ( clipRegion.IsEmpty() )
+    {
+        m_clipX1 = m_clipY1 = m_clipX2 = m_clipY2 = 0;
+    }
+    else
+    {
+        m_clipX1 = clipRegion.GetLeftTop().x;
+        m_clipY1 = clipRegion.GetLeftTop().y;
+        m_clipX2 = clipRegion.GetBottomRight().x + 1;
+        m_clipY2 = clipRegion.GetBottomRight().y + 1;
     }
 }