Fix NumericUpDown button rendering regression when visual styles are disabled

[LeafShi1]

Fixes #14641

Root Cause

The non-visual-style path was unintentionally routed to the modern button renderer by this condition:
Application.IsDarkModeEnabled || !Application.RenderWithVisualStyles

That behavior came from PR #13747.

Proposed changes

In UpDownBase.UpDownButtons.cs:

• Use modern renderer only when dark mode is enabled.
• Keep themed renderer path when visual styles are enabled.
• Restore classic DrawScrollButton fallback when visual styles are disabled.

Customer Impact

• After fix, the up/down buttons of the NumericUpDown control are normal gray color.

Regression?

• Yes

Risk

• Minimal

Screenshots

Before

The up/down buttons of the NumericUpDown control are black.

After

The up/down buttons of the NumericUpDown control are gray color.

Test methodology

• Manually

Test environment(s)

• .net 11.0.0-preview.6.26311.113

Microsoft Reviewers: Open in CodeFlow

[Copilot]

Pull request overview

Fixes a rendering regression in NumericUpDown/UpDownBase where the up/down buttons were drawn using the modern (dark-mode) renderer even when visual styles were disabled, resulting in incorrect (black) buttons in classic mode.

Changes:

• Restricts the modern control-button renderer to dark mode only (Application.IsDarkModeEnabled).
• Uses VisualStyleRenderer only when visual styles are enabled (Application.RenderWithVisualStyles).
• Restores classic ControlPaint.DrawScrollButton rendering when visual styles are disabled.

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