[WinForms] Incorrect Tree Nodes drawing at the `OwnerDrawText` mode (#18605)

Assume we have the following form `MyForm`:
```
using System;
using System.Drawing;
using System.Windows.Forms;

namespace test_TreeView
{
    public class MyForm : Form
    {
        TreeView treeView = new TreeView()

        public MyForm()
        {
            treeView.Nodes.Add("root node");
            treeView.Nodes[0].Nodes.Add("first child node");

            treeView.BackColor = Color.Gray;
            treeView.LineColor = Color.Yellow;

            treeView.DrawMode = TreeViewDrawMode.OwnerDrawText;
            treeView.DrawNode += treeView_DrawNode;

            Controls.Add(treeView);
        }

        private void treeView_DrawNode(object sender, DrawTreeNodeEventArgs e)
        {
            var brush = new SolidBrush(Color.FromArgb(100, 255, 0, 0));
            e.Graphics.FillRectangle(brush, e.Bounds);
        }
    }
}
```
The displayed result is
![image](https://user-images.githubusercontent.com/12382656/73263417-92872980-41e1-11ea-8f7c-7cecbb376b60.png)
One can see couple drawing issues:
* White rectangles under each `TreeNode`.
* The area of custom drawing (red rectangles) is shifted with respect to this white rectangles.

Current PR proposes the fix of this issues.
Co-authored-by: monojenkins <jo.shields+jenkins@xamarin.com>

3 files changed, 17 insertions, 18 deletions

diff --git a/mcs/class/System.Windows.Forms/System.Windows.Forms/Control.cs b/mcs/class/System.Windows.Forms/System.Windows.Forms/Control.cs
index d85c52dff6e..53f489d751b 100644
--- a/mcs/class/System.Windows.Forms/System.Windows.Forms/Control.cs
+++ b/mcs/class/System.Windows.Forms/System.Windows.Forms/Control.cs
@@ -1406,8 +1406,7 @@ namespace System.Windows.Forms
 			if (background_image == null) {
 				if (!tbstyle_flat) {
 					Rectangle paintRect = pevent.ClipRectangle;
-					Brush pen = ThemeEngine.Current.ResPool.GetSolidBrush(BackColor);
-					pevent.Graphics.FillRectangle(pen, paintRect);
+					pevent.Graphics.FillRectangle(BackColorBrush, paintRect);
 				}
 				return;
 			}
@@ -1417,8 +1416,8 @@ namespace System.Windows.Forms
 
 		void DrawBackgroundImage (Graphics g) {
 			Rectangle drawing_rectangle = new Rectangle ();
-			g.FillRectangle (ThemeEngine.Current.ResPool.GetSolidBrush (BackColor), ClientRectangle);
-				
+			g.FillRectangle (BackColorBrush, ClientRectangle);
+			
 			switch (backgroundimage_layout)
 			{
 			case ImageLayout.Tile:
@@ -3469,6 +3468,8 @@ namespace System.Windows.Forms
 			}
 		}
 
+		protected SolidBrush BackColorBrush => ThemeEngine.Current.ResPool.GetSolidBrush (BackColor);
+
 		#endregion	// Protected Instance Properties
 
 		#region Public Static Methods
diff --git a/mcs/class/System.Windows.Forms/System.Windows.Forms/ControlPaint.cs b/mcs/class/System.Windows.Forms/System.Windows.Forms/ControlPaint.cs
index 4004c1ec8ec..5a933f1dc10 100644
--- a/mcs/class/System.Windows.Forms/System.Windows.Forms/ControlPaint.cs
+++ b/mcs/class/System.Windows.Forms/System.Windows.Forms/ControlPaint.cs
@@ -370,11 +370,11 @@ namespace System.Windows.Forms {
 			ThemeEngine.Current.CPDrawContainerGrabHandle (graphics, bounds);
 		}
 
-		public static void DrawFocusRectangle( Graphics graphics, Rectangle rectangle) {
+		public static void DrawFocusRectangle(Graphics graphics, Rectangle rectangle) {
 			DrawFocusRectangle(graphics, rectangle, SystemColors.Control, SystemColors.ControlText);
 		}
 
-		public static void DrawFocusRectangle( Graphics graphics, Rectangle rectangle, Color foreColor, Color backColor) {
+		public static void DrawFocusRectangle(Graphics graphics, Rectangle rectangle, Color foreColor, Color backColor) {
 
 			ThemeEngine.Current.CPDrawFocusRectangle (graphics, rectangle, foreColor, backColor);
 		}
diff --git a/mcs/class/System.Windows.Forms/System.Windows.Forms/TreeView.cs b/mcs/class/System.Windows.Forms/System.Windows.Forms/TreeView.cs
index 74d77f5f80f..a66f82ccc36 100644
--- a/mcs/class/System.Windows.Forms/System.Windows.Forms/TreeView.cs
+++ b/mcs/class/System.Windows.Forms/System.Windows.Forms/TreeView.cs
@@ -1419,7 +1419,7 @@ namespace System.Windows.Forms {
 
 		private void Draw (Rectangle clip, Graphics dc)
 		{
-			dc.FillRectangle (ThemeEngine.Current.ResPool.GetSolidBrush (BackColor), clip);
+			dc.FillRectangle (BackColorBrush, clip);
 
 			if (dash == null)
 				CreateDashPen ();
@@ -1670,7 +1670,6 @@ namespace System.Windows.Forms {
 		{
 			if (!full_row_select || show_lines)
 				DrawSelectionAndFocus(node, dc, node.Bounds);
-
 			
 			Font font = node.NodeFont;
 			if (node.NodeFont == null)
@@ -1691,7 +1690,7 @@ namespace System.Windows.Forms {
 			int middle = y + (ActualItemHeight / 2);
 
 			if (full_row_select && !show_lines) {
-				Rectangle r = new Rectangle (1, y, ViewportRectangle.Width - 2, ActualItemHeight);
+				var r = new Rectangle (1, y, ViewportRectangle.Width - 2, ActualItemHeight);
 				DrawSelectionAndFocus (node, dc, r);
 			}
 
@@ -1716,26 +1715,25 @@ namespace System.Windows.Forms {
 			}
 
 			if (draw_mode != TreeViewDrawMode.Normal) {
-				dc.FillRectangle (Brushes.White, node.Bounds);
-				TreeNodeStates tree_node_state = TreeNodeStates.Default;;
+				dc.FillRectangle (BackColorBrush, node.Bounds);
+
+				var tree_node_state = TreeNodeStates.Default;;
 				if (node.IsSelected)
 					tree_node_state = TreeNodeStates.Selected;
 				if (node.Checked)
 					tree_node_state |= TreeNodeStates.Checked;
 				if (node == focused_node)
 					tree_node_state |= TreeNodeStates.Focused;
-				Rectangle node_bounds = node.Bounds;
-				if (draw_mode == TreeViewDrawMode.OwnerDrawText) {
-					node_bounds.X += 3;
-					node_bounds.Y += 1;
-				} else {
+				
+				var node_bounds = node.Bounds;
+				if (draw_mode != TreeViewDrawMode.OwnerDrawText) {
 					node_bounds.X = 0;
 					node_bounds.Width = Width;
 				}
 
-				DrawTreeNodeEventArgs e = new DrawTreeNodeEventArgs (dc, node, node_bounds, tree_node_state);
+				var e = new DrawTreeNodeEventArgs (dc, node, node_bounds, tree_node_state);
 
-				OnDrawNode (e);				
+				OnDrawNode (e);
 				if (!e.DrawDefault)
 					return;
 			}