Process HDN_ITEMCHANGING notifications only when column is being resized

When column resizing is finished, after HDN_ENDTRACK notification there is
also sent one (and last) HDN_ITEMCHANGING notification. We have to skip it
to prevent from sending EVT_HEADER_RESIZING after EVT_HEADER_END_RESIZE
because EVT_HEADER_END_RESIZE should be really the last one event
in the sequence of resizing events (like it's assumed in wxGrid).

Closes #16390.

include/wx/msw/headerctrl.h

@@ -139,6 +139,9 @@ private:
     // actual column we are dragging or -1 if not dragging anything
     int m_colBeingDragged;
 
+    // a column is currently being resized
+    bool m_isColBeingResized;
+
     // the custom draw helper: initially NULL, created on demand, use
     // GetCustomDraw() to do it
     wxMSWHeaderCtrlCustomDraw *m_customDraw;

src/msw/headerctrl.cpp

@@ -95,6 +95,7 @@ void wxHeaderCtrl::Init()
     m_imageList = NULL;
     m_scrollOffset = 0;
     m_colBeingDragged = -1;
+    m_isColBeingResized = false;
     m_customDraw = NULL;
 }
 
@@ -695,8 +696,9 @@ bool wxHeaderCtrl::MSWOnNotify(int idCtrl, WXLPARAM lParam, WXLPARAM *result)
                 break;
             }
 
+            m_isColBeingResized = true;
             evtType = wxEVT_HEADER_BEGIN_RESIZE;
-            // fall through
+            wxFALLTHROUGH;
 
         case HDN_ENDTRACKA:
         case HDN_ENDTRACKW:
@@ -710,6 +712,8 @@ bool wxHeaderCtrl::MSWOnNotify(int idCtrl, WXLPARAM lParam, WXLPARAM *result)
                 const int minWidth = GetColumn(idx).GetMinWidth();
                 if ( width < minWidth )
                     width = minWidth;
+
+                m_isColBeingResized = false;
             }
             break;
 
@@ -719,7 +723,31 @@ bool wxHeaderCtrl::MSWOnNotify(int idCtrl, WXLPARAM lParam, WXLPARAM *result)
             // just in case we are dealing with one of these buggy versions.
         case HDN_TRACK:
         case HDN_ITEMCHANGING:
-            if ( nmhdr->pitem && (nmhdr->pitem->mask & HDI_WIDTH) )
+            // With "Show window contents while dragging" option enabled
+            // the sequence of notifications is as follows:
+            //   HDN_BEGINTRACK
+            //   HDN_ITEMCHANGING
+            //   HDN_ITEMCHANGED
+            //   ...
+            //   HDN_ITEMCHANGING
+            //   HDN_ITEMCHANGED
+            //   HDN_ENDTRACK
+            //   HDN_ITEMCHANGING
+            //   HDN_ITEMCHANGED
+            // With "Show window contents while dragging" option disabled
+            // the sequence looks in turn like this:
+            //   HDN_BEGINTRACK
+            //   HDN_ITEMTRACK
+            //   HDN_ITEMCHANGING
+            //   ...
+            //   HDN_ITEMTRACK
+            //   HDN_ITEMCHANGING
+            //   HDN_ENDTRACK
+            //   HDN_ITEMCHANGING
+            //   HDN_ITEMCHANGED
+            // In both cases last HDN_ITEMCHANGING notification is sent
+            // after HDN_ENDTRACK so we have to skip it.
+            if ( nmhdr->pitem && (nmhdr->pitem->mask & HDI_WIDTH) && m_isColBeingResized )
             {
                 // prevent the column from being shrunk beneath its min width
                 width = nmhdr->pitem->cxy;