Fix wxTreeListCtrl under wxOSX

Postpone resetting indent in wxOSX wxDataViewCtrl to avoid always removing,
and hence never showing the expanders at all, for wxTreeListCtrl whose model
starts its life as a list but becomes a tree as soon as any items with
children are added to it.

By postponing the call to IsListModel() until the next resize, we give the
model the time it needs to decide what it's going to be, while still removing
the unnecessary indent if there is no need for it.

Closes #17409.

include/wx/osx/cocoa/dataview.h

@@ -534,6 +534,8 @@ private:
     wxCocoaOutlineDataSource* m_DataSource;
 
     wxCocoaOutlineView* m_OutlineView;
+
+    bool m_removeIndentIfNecessary;
 };
 
 #endif // _WX_DATAVIEWCTRL_COCOOA_H_

src/osx/cocoa/dataview.mm

@@ -1939,7 +1939,8 @@ wxCocoaDataViewControl::wxCocoaDataViewControl(wxWindow* peer,
         [[NSScrollView alloc] initWithFrame:wxOSXGetFrameForControl(peer,pos,size)]
       ),
       m_DataSource(NULL),
-      m_OutlineView([[wxCocoaOutlineView alloc] init])
+      m_OutlineView([[wxCocoaOutlineView alloc] init]),
+      m_removeIndentIfNecessary(false)
 {
     // initialize scrollview (the outline view is part of a scrollview):
     NSScrollView* scrollview = (NSScrollView*) GetWXWidget();
@@ -2289,13 +2290,11 @@ bool wxCocoaDataViewControl::AssociateModel(wxDataViewModel* model)
         m_DataSource = NULL;
     [m_OutlineView setDataSource:m_DataSource]; // if there is a data source the data is immediately going to be requested
 
-    // By default, the first column is indented to leave enough place for the
+    // Set this to true to check if we need to remove the indent in the next
-    // expanders, but this looks bad if there are no expanders, so don't use
+    // OnSize() call: we can't do it directly here because the model might not
-    // indent in this case.
+    // be fully initialized yet and so might not know whether it has any items
-    if ( model && model->IsListModel() )
+    // with children or not.
-    {
+    m_removeIndentIfNecessary = true;
-        DoSetIndent(0);
-    }
 
     return true;
 }
@@ -2460,6 +2459,21 @@ void wxCocoaDataViewControl::SetRowHeight(const wxDataViewItem& WXUNUSED(item),
 
 void wxCocoaDataViewControl::OnSize()
 {
+    if ( m_removeIndentIfNecessary )
+    {
+        m_removeIndentIfNecessary = false;
+
+        const wxDataViewModel* const model = GetDataViewCtrl()->GetModel();
+
+        // By default, the first column is indented to leave enough place for the
+        // expanders, but this looks bad if there are no expanders, so don't use
+        // indent in this case.
+        if ( model && model->IsListModel() )
+        {
+            DoSetIndent(0);
+        }
+    }
+
     if ([m_OutlineView numberOfColumns] == 1)
         [m_OutlineView sizeLastColumnToFit];
 }