Fix horizontal scrollbar handling for wxCheckListBox in wxMSW

The scrollbar wasn't shown when the control became only slightly more
narrow than its contents, resulting in truncation of the rightmost part
of the strings shown in it.

Fix this by accounting for the check box explicitly in wxListBox
SetHorizontalExtent() method using the new MSWGetFullItemSize() helper
which is also used to avoid code duplication between wxCheckListBox
MSWOnMeasure() and DoGetBestClientSize() methods.

Closes #18377.

include/wx/msw/checklst.h

@@ -65,6 +65,8 @@ public:
     virtual bool MSWOnMeasure(WXMEASUREITEMSTRUCT *item) wxOVERRIDE;
 
 protected:
+    virtual wxSize MSWGetFullItemSize(int w, int h) const wxOVERRIDE;
+
     // pressing space or clicking the check box toggles the item
     void OnKeyDown(wxKeyEvent& event);
     void OnLeftClick(wxMouseEvent& event);

include/wx/msw/listbox.h

@@ -174,6 +174,13 @@ protected:
     // this can't be called DoHitTest() because wxWindow already has this method
     virtual int DoHitTestList(const wxPoint& point) const;
 
+    // This is a hook for wxCheckListBox, which uses it to add the checkbox
+    // width to the item width and to make it at least as tall as the checkbox.
+    virtual wxSize MSWGetFullItemSize(int w, int h) const
+    {
+        return wxSize(w, h);
+    }
+
     // free memory (common part of Clear() and dtor)
     void Free();
 

src/msw/checklst.cpp

@@ -246,15 +246,10 @@ bool wxCheckListBox::MSWOnMeasure(WXMEASUREITEMSTRUCT *item)
     {
         MEASUREITEMSTRUCT *pStruct = (MEASUREITEMSTRUCT *)item;
 
-        wxSize size = wxRendererNative::Get().GetCheckBoxSize(this);
-        size.x += 2 * CHECKMARK_EXTRA_SPACE;
-        size.y += 2 * CHECKMARK_EXTRA_SPACE;
-
-        // add place for the check mark
-        pStruct->itemWidth += size.GetWidth();
-
-        if ( pStruct->itemHeight < static_cast<unsigned int>(size.GetHeight()) )
-            pStruct->itemHeight = size.GetHeight();
+        const wxSize size = MSWGetFullItemSize(pStruct->itemWidth,
+                                                 pStruct->itemHeight);
+        pStruct->itemWidth = size.x;
+        pStruct->itemHeight = size.y;
 
         return true;
     }
@@ -422,20 +417,25 @@ void wxCheckListBox::OnLeftClick(wxMouseEvent& event)
     }
 }
 
+wxSize wxCheckListBox::MSWGetFullItemSize(int w, int h) const
+{
+    wxSize size = wxRendererNative::Get().GetCheckBoxSize(const_cast<wxCheckListBox*>(this));
+    size.x += 2 * CHECKMARK_EXTRA_SPACE;
+    size.y += 2 * CHECKMARK_EXTRA_SPACE;
+
+    w += size.GetWidth();
+    if ( h < size.GetHeight() )
+        h = size.GetHeight();
+
+    return wxSize(w, h);
+}
+
 wxSize wxCheckListBox::DoGetBestClientSize() const
 {
     wxSize best = wxListBox::DoGetBestClientSize();
 
     // add room for the checkbox
-    wxSize size = wxRendererNative::Get().GetCheckBoxSize(const_cast<wxCheckListBox*>(this));
-    size.x += 2 * CHECKMARK_EXTRA_SPACE;
-    size.y += 2 * CHECKMARK_EXTRA_SPACE;
-
-    best.x += size.GetWidth();
-    if ( best.y < size.GetHeight() )
-        best.y = size.GetHeight();
-
-    return best;
+    return MSWGetFullItemSize(best.x, best.y);
 }
 
 #endif // wxUSE_CHECKLISTBOX

src/msw/listbox.cpp

@@ -596,7 +596,10 @@ void wxListBox::SetHorizontalExtent(const wxString& s)
     }
 
     if ( largestExtent )
+    {
+        largestExtent = MSWGetFullItemSize(largestExtent, 0 /* height */).x;
         SendMessage(GetHwnd(), LB_SETHORIZONTALEXTENT, LOWORD(largestExtent), 0L);
+    }
     //else: it shouldn't change
 }