Fix completely broken default button handling in wxMSW

The logic in wxButton::{Set,Unset}TmpDefault() didn't work at all when
the temporary button was the same as the permanent default button as the
code made the same button non-default immediately after making it
default (or vice versa). In particular, this ensured that default button
was never highlighted correctly (at least after the first focus change)
in dialogs containing a single button only.

Fix this by simply skipping modification of the old default button if it
was the same one as the new default button.

docs/changes.txt

@@ -172,6 +172,7 @@ wxMSW:
 - Fix wxScreenDC::GetSize() with multiple monitors (iwbnwif).
 - Fix background colour returned by wxTextCtrl::GetStyle() (Andreas Falkenhahn).
 - Revert to using equally-sized buttons in wxToolBar by default.
+- Fix default button highlighting.
 - Restore dispatching wxThreadEvent while resizing the window broken in 3.1.0.
 - Fix wxGraphicsMatrix::TransformDistance for Direct2D renderer.
 - Fix wxDC::Clear() for rotated DC.

src/msw/button.cpp

@@ -284,13 +284,19 @@ void wxButton::SetTmpDefault()
         return;
 
     wxWindow *winOldDefault = tlw->GetDefaultItem();
+
     tlw->SetTmpDefaultItem(this);
 
     // Notice that the order of these statements is important, the old button
     // is not reset if we do it the other way round, probably because of
     // something done by the default DM_SETDEFID handler.
     SetDefaultStyle(this, true);
-    SetDefaultStyle(wxDynamicCast(winOldDefault, wxButton), false);
+    if ( winOldDefault != this )
+    {
+        // But we mustn't reset the default style on this button itself if it
+        // had already been the default.
+        SetDefaultStyle(wxDynamicCast(winOldDefault, wxButton), false);
+    }
 }
 
 // unset this button as currently default, it may still stay permanent default
@@ -306,7 +312,10 @@ void wxButton::UnsetTmpDefault()
 
     // Just as in SetTmpDefault() above, the order is important here.
     SetDefaultStyle(wxDynamicCast(winOldDefault, wxButton), true);
-    SetDefaultStyle(this, false);
+    if ( winOldDefault != this )
+    {
+        SetDefaultStyle(this, false);
+    }
 }
 
 /* static */