Fix setting wxBU_EXACTFIT style for wxButton under wxGTK2

The way wxBU_EXACTFIT is implemented in commit c1bb80987f
("Improve implementation of wxBU_EXACTFIT style for wxButton under wxGTK2",
2020-04-12) is too intrusive and disrupts button's appearance in some
themes. It should work better with themes if we reduce inner border in
a more GTK-compliant way by applying a dedicated GTK style to the button.

Closes #19081.

include/wx/gtk/button.h

@@ -67,6 +67,12 @@ private:
     // Return the GtkLabel used by this button.
     GtkLabel *GTKGetLabel() const;
 
+#ifndef __WXGTK3__
+    // To mark if special GTK style for buttons with wxBU_EXACTFIT flag
+    // was already defined.
+    static bool m_exactFitStyleDefined;
+#endif // !__WXGTK3__
+
     wxDECLARE_DYNAMIC_CLASS(wxButton);
 };
 

src/gtk/button.cpp

@@ -70,6 +70,10 @@ wxgtk_button_style_set_callback(GtkWidget* widget, GtkStyle*, wxButton* win)
 // wxButton
 //-----------------------------------------------------------------------------
 
+#ifndef __WXGTK3__
+bool wxButton::m_exactFitStyleDefined = false;
+#endif // !__WXGTK3__
+
 bool wxButton::Create(wxWindow *parent,
                       wxWindowID id,
                       const wxString &label,
@@ -141,7 +145,20 @@ bool wxButton::Create(wxWindow *parent,
 #ifdef __WXGTK3__
         GTKApplyCssStyle("* { padding:0 }");
 #else
-        GTKApplyWidgetStyle(true); // To enforce call to DoApplyWidgetStyle()
+        // Define a special button style without inner border
+        // if it's not yet done.
+        if ( !m_exactFitStyleDefined )
+        {
+            gtk_rc_parse_string(
+              "style \"wxButton_wxBU_EXACTFIT_style\"\n"
+              "{ GtkButton::inner-border = { 0, 0, 0, 0 } }\n"
+              "widget \"*wxButton_wxBU_EXACTFIT*\" style \"wxButton_wxBU_EXACTFIT_style\"\n"
+            );
+            m_exactFitStyleDefined = true;
+        }
+
+        // Assign the button to the GTK style without inner border.
+        gtk_widget_set_name(m_widget, "wxButton_wxBU_EXACTFIT");
 #endif // __WXGTK3__ / !__WXGTK3__
     }
 
@@ -312,11 +329,6 @@ GtkLabel *wxButton::GTKGetLabel() const
 
 void wxButton::DoApplyWidgetStyle(GtkRcStyle *style)
 {
-    if ( style && HasFlag(wxBU_EXACTFIT) )
-    {
-        style->xthickness = 0;
-        style->ythickness = 0;
-    }
     GTKApplyStyle(m_widget, style);
     GtkWidget* child = gtk_bin_get_child(GTK_BIN(m_widget));
     GTKApplyStyle(child, style);