Regression: ToolStrip dropdown scrolling stops one item too early when navigating with Down Arrow

### .NET version

.NET 11 SDK build: 11.0.100-preview.5.26256.117

### Did it work in .NET Framework?

Yes

### Did it work in any of the earlier releases of .NET Core or .NET 5+?

Yes, it doesn't repro in .NET 10 and previous .NET versions, regressed from the PR change: https://github.com/dotnet/winforms/pull/14490

### Issue description

When the dropdown can display last two or several items, keyboard navigation using the Down Arrow key stops one step earlier than before.

**Current Behavior:**
Once the last two items become visible (for example C and D), the bottom scroll arrow becomes disabled immediately.
Further Down Arrow navigation is no longer possible.

**Expected Behavior:**
Navigation should continue one more step.
Previously, after C and D became visible, pressing Down again would scroll further so that only the final item (D) remained visible.
The bottom scroll arrow should only become disabled after the final item is fully reached.

### Steps to reproduce

1. Create a ToolStripDropDownButton (or similar dropdown control) with multiple items.
2. Resize/configure the dropdown so only two items are visible at a time.
3. Open the dropdown.
4. Press the Down Arrow key repeatedly to navigate through the items