Allow selecting and copying text in wxMessageDialog on GTK

Normally the GTK message dialogs have non-selectable text, but that also
means users can't copy the text. This can be inconvenient if the dialog
is used for error messages, since then the error message can't be
copied.

Instead, make the text labels used in the GTK message dialog selectable
to fix that.

See #23039.

(cherry picked from commit a8ea3eda116ae895446fc720943702f0c9c52b90)

docs/changes.txt

@@ -268,6 +268,7 @@ wxGTK:
 - Fix compilation with GTK 3.22.z and z < 25 (#22816).
 - Fix compilation with GTK < 2.21.8 (mkirkham, #22830).
 - Fix display artefacts when using AUI without compositor under X11 (#23135).
+- Allow selecting and copying text in wxMessageDialog (Ian McInerney, #23039).
 
 wxMSW:
 

src/gtk/msgdlg.cpp

@@ -23,6 +23,7 @@
 #include "wx/modalhook.h"
 
 #include "wx/gtk/private.h"
+#include "wx/gtk/private/list.h"
 #include "wx/gtk/private/messagetype.h"
 #include "wx/gtk/private/mnemonics.h"
 #include "wx/gtk/private/dialogcount.h"
@@ -187,6 +188,26 @@ void wxMessageDialog::GTKCreateMsgDialog()
         gtk_window_set_keep_above(GTK_WINDOW(m_widget), TRUE);
     }
 
+    // A GTKMessageDialog usually displays its labels without selection enabled,
+    // so we enable selection to allow the user to select+copy the text out of
+    // the dialog.
+    {
+        GtkMessageDialog * const msgdlg = GTK_MESSAGE_DIALOG(m_widget);
+
+        GtkWidget* const area = gtk_message_dialog_get_message_area(msgdlg);
+        wxGtkList labels(gtk_container_get_children(GTK_CONTAINER(area)));
+
+        for ( GList* elem = labels; elem; elem = elem->next )
+        {
+            GtkWidget* const widget = GTK_WIDGET( elem->data );
+
+            if ( GTK_IS_LABEL(widget) )
+            {
+                gtk_label_set_selectable(GTK_LABEL(widget), TRUE);
+            }
+        }
+    }
+
     // we need to add buttons manually if we use custom labels or always for
     // Yes/No/Cancel dialog as GTK+ doesn't support it natively
     const bool addButtons = buttons == GTK_BUTTONS_NONE;