When the user presses the TAB key, the input focus is set to the next control in the tab order of the form. Set SplitContainer.TabStop to true to give input focus to a splitter so that it can be moved with the arrow keys as well as with the mouse. Starting in the net_v40_long, setting SplitContainer.TabStop to false excludes the splitter and any of the controls that are contained in the System.Windows.Forms.SplitContainer from the collection of controls in the tab order. To enable controls to get focus by using the TAB key, create a control that inherits from System.Windows.Forms.SplitContainer. Create a new property named TabStop and override the SplitContainer.ProcessTabKey(bool) method. The following example demonstrates how to accomplish this.

code reference: SplitContainerTabStop#1

You can manipulate the tab order by setting the control's Control.TabIndex property value.