Fix bug in CanUndo() returning true after wxTextCtrl creation

For wxMSW text controls with wxTE_RICH2 style, calling SetFont() counts
as an undoable operation, resulting in CanUndo() returning true even if
no "real" changes have been made yet.

Fix this by resetting the undo stack after creating the control using
ITextDocument::Undo().

Unfortunately this interface is not available in MinGW-32, so this fix
can't be used with it.

Closes https://github.com/wxWidgets/wxWidgets/pull/2010

Closes #17524.

src/msw/textctrl.cpp

@@ -65,7 +65,21 @@
 #if wxUSE_RICHEDIT
     #include <richedit.h>
     #include <richole.h>
+
+    // MinGW32 doesn't have tom.h and doesn't define the interfaces and the
+    // constants we need, so we can't use ITextDocument::Undo() with it. All
+    // the other compilers do have this header.
+    #ifndef __MINGW32_TOOLCHAIN__
+        #define wxHAS_TOM_H
+    #endif
+
+    #ifdef wxHAS_TOM_H
+        #include <tom.h>
+    #endif
+
     #include "wx/msw/ole/oleutils.h"
+
+    #include "wx/msw/private/comptr.h"
 #endif // wxUSE_RICHEDIT
 
 #if wxUSE_INKEDIT
@@ -129,6 +143,17 @@ DEFINE_GUID(wxIID_IRichEditOleCallback,
     0x00020d03, 0x0000, 0x0000, 0xc0, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x46);
 
 } // anonymous namespace
+
+#ifdef wxHAS_TOM_H
+
+// This one is not defined in the standard libraries at all and MSDN just says
+// to define it explicitly, so we do it for IID_XXX constant itself and not our
+// own wxIID_XXX.
+DEFINE_GUID(IID_ITextDocument,
+    0x8cc497c0, 0xa1df, 0x11ce, 0x80, 0x98, 0x00, 0xaa, 0x00, 0x47, 0xbe, 0x5d);
+
+#endif // wxHAS_TOM_H
+
 #endif // wxUSE_OLE
 
 // ----------------------------------------------------------------------------
@@ -668,6 +693,34 @@ bool wxTextCtrl::MSWCreateText(const wxString& value,
         ::SendMessage(GetHwnd(), EM_SETMARGINS, wParam, lParam);
     }
 
+#if wxUSE_RICHEDIT && defined(wxHAS_TOM_H)
+    // For RichEdit >= 4, SetFont(), called above from MSWCreateControl(), uses
+    // EM_SETCHARFORMAT which affects the undo buffer, meaning that CanUndo()
+    // for a newly created control returns true, which is unexpected. To avoid
+    // this, we explicitly use Undo(tomFalse) here to clear the undo buffer.
+    // And since Undo(tomFalse) also disables the undo buffer, we need to
+    // enable it again immediately after clearing by calling Undo(tomTrue).
+    if ( GetRichVersion() >= 4 )
+    {
+        wxCOMPtr<IRichEditOle> pRichEditOle;
+        if ( SendMessage(GetHwnd(), EM_GETOLEINTERFACE,
+                         0, (LPARAM)&pRichEditOle) && pRichEditOle )
+        {
+            wxCOMPtr<ITextDocument> pDoc;
+            HRESULT hr = pRichEditOle->QueryInterface
+                                       (
+                                        wxIID_PPV_ARGS(ITextDocument, &pDoc)
+                                       );
+            if ( SUCCEEDED(hr) )
+            {
+                hr = pDoc->Undo(tomFalse, NULL);
+                if ( SUCCEEDED(hr) )
+                    pDoc->Undo(tomTrue, NULL);
+            }
+        }
+    }
+#endif // wxUSE_RICHEDIT && wxHAS_TOM_H
+
     return true;
 }
 

tests/controls/textctrltest.cpp

@@ -1432,4 +1432,35 @@ TEST_CASE("wxTextCtrl::EventsOnCreate", "[wxTextCtrl][event]")
     CHECK( updated.GetCount() == 1 );
 }
 
+TEST_CASE("wxTextCtrl::InitialCanUndo", "[wxTextCtrl][undo]")
+{
+    wxWindow* const parent = wxTheApp->GetTopWindow();
+
+    const long styles[] = { 0, wxTE_RICH, wxTE_RICH2 };
+
+    for ( size_t n = 0; n < WXSIZEOF(styles); n++ )
+    {
+        const long style = styles[n];
+
+#ifdef __MINGW32_TOOLCHAIN__
+        if ( style == wxTE_RICH2 )
+        {
+            // We can't call ITextDocument::Undo() in wxMSW code when using
+            // MinGW32, so this test would always fail with it.
+            WARN("Skipping test known to fail with MinGW-32.");
+        }
+        continue;
+#endif // __MINGW32_TOOLCHAIN__
+
+        INFO("wxTextCtrl with style " << style);
+
+        wxScopedPtr<wxTextCtrl> text(new wxTextCtrl(parent, wxID_ANY, "",
+                                                    wxDefaultPosition,
+                                                    wxDefaultSize,
+                                                    style));
+
+        CHECK( !text->CanUndo() );
+    }
+}
+
 #endif //wxUSE_TEXTCTRL