title: TabIndex does not work correctly
body: <p>I have a windows form application. On the form there are three groupboxs.
  Each groupbox contains some controls. Please see the image.
  <img src="https://i.stack.imgur.com/3mz6v.png" alt="form"></p>
  
  <p>There is a groupbox "flag" that contains a few checkboxs. "flag" is inside in "groupbox1".
  I used Tab key to go through each control but it doesn't work for checkboxs in "flag". I did set proper tabindex for each control.</p>
  
  <p>It works for textboxs and buttons but checkboxs.</p>
  
  <p>Why? Thanks for help.</p>
  
  <p><strong>EDIT</strong></p>
  
  <pre><code> // groupBox2
          // 
          this.groupBox2.Controls.Add(this.pictureBox10);
          this.groupBox2.Controls.Add(this.pictureBox9);
          this.groupBox2.Controls.Add(this.pictureBox8);
          this.groupBox2.Controls.Add(this.pictureBox7);
          this.groupBox2.Controls.Add(this.chkStoplight);
          this.groupBox2.Controls.Add(this.lblStoplight);
          this.groupBox2.Controls.Add(this.chkIsCount);
          this.groupBox2.Controls.Add(this.chkExceptionFlag);
          this.groupBox2.Controls.Add(this.chkIsActive);
          this.groupBox2.Controls.Add(this.lblIsActive);
          this.groupBox2.Controls.Add(this.lblExceptionFlag);
          this.groupBox3.Controls.Add(this.lblIsCount);
          this.groupBox2.Location = new System.Drawing.Point(16, 201);
          this.groupBox2.Name = "groupBox2";
          this.groupBox2.Size = new System.Drawing.Size(321, 70);
          this.groupBox2.TabIndex = 10;
          this.groupBox2.TabStop = true;
          this.groupBox2.Text = "Flags";
  
          // 
          // chkStoplight
          // 
          this.chkStoplight.AutoSize = true;
          this.chkStoplight.Location = new System.Drawing.Point(44, 25);
          this.chkStoplight.Name = "chkStoplight";
          this.chkStoplight.Size = new System.Drawing.Size(15, 14);
          this.chkStoplight.TabIndex = 0;
          this.chkStoplight.UseVisualStyleBackColor = true;
  
          In the property, I found TabStop is true for chkStoplight.
  </code></pre>