Fix ToolStripDropDownMenu scrolling when items are unavailable

[SweathaBharathi]

Fixes #7635

Proposed changes

• Updated ToolStripDropDownMenu.ScrollInternal(bool up) to skip items that are not available when calculating the scroll target and scroll delta.
• Added helper logic to find the previous/next visible item instead of directly using adjacent items from the raw Items collection.
• Fixed scroll boundary issues caused by hidden items (Available = false), including empty space at the top or bottom, overscrolling at the end of the list, and looping behavior when scrolling upward.

Customer Impact

• Fixes incorrect ContextMenuStrip / ToolStripDropDownMenu scrolling when some items are hidden via Available = false, which previously could produce empty space at the top or bottom while scrolling.
• Fixes boundary issues where scrolling could continue past the last visible item or fail to return to the first visible item when hidden items were present.

Regression?

• No

Risk

• No

Screenshots

Before

After

Test methodology

• Reproduced the original issue using a ContextMenuStrip with MaximumSize set small enough to enable scrolling and with one item added with Available = false before being inserted into the menu.
• Verified scrolling behavior with hidden/unavailable items in the middle, at the first position, and at the last position of the collection.
• Confirmed there is no extra empty space at the top or bottom, no incorrect growth at the scroll boundary, and no looping when scrolling upward.
• Verified that menus without hidden/unavailable items still scroll normally and that both scroll buttons update correctly while moving through the visible item range.

Accessibility testing

• This change does not introduce new controls or alter keyboard navigation semantics; it only corrects scroll positioning when hidden/unavailable items exist.

Test environment(s)

• OS: Windows
• SDK: 11.0.100-preview.3.26170.106
• UI scaling: 100% / 150% / 125 % / 175%

Microsoft Reviewers: Open in CodeFlow