How to: Manage ToolStrip Overflow in Windows Forms
When you add ToolStripItems that require more space than is allotted to the ToolStrip given the form's current size, a ToolStripOverflowButton automatically appears on the ToolStrip. The ToolStripOverflowButton appears, and overflow-enabled items are moved into the drop-down overflow menu. This allows you to customize and prioritize how your ToolStrip items properly adjust to different form sizes. You can also change the appearance of your items when they fall into the overflow by using the Placement and ToolStripOverflow.DisplayedItems properties and the LayoutCompleted event. If you enlarge the form at either design time or run time, more ToolStripItems can be displayed on the main ToolStrip and the ToolStripOverflowButton might even disappear until you decrease the size of the form.
To enable overflow on a ToolStrip control
When CanOverflow is True (the default), a ToolStripItem is sent to the drop-down overflow menu when the content of the ToolStripItem exceeds the width of a horizontal ToolStrip or the height of a vertical ToolStrip.