Fix editable control border color in NET11 visual styles

[LeafShi1]

Fixes #14847

Proposed changes

Introduce shared editable-control border color logic in ModernControlColorMath and use it across NET11 editable renderers.

• Uses a stable border color instead of ForeColor.
• Applies the shared border color to TextBoxBase, ComboBox.ModernComboAdapter, and UpDownBase.
• Uses a visible dark-mode border color.

Customer Impact

• Customers using VisualStylesMode.Net11 can customize editable-control text colors without unintentionally changing the control border.

Regression?

• No

Risk

• Minimal

Screenshots

Before

Changing ForeColor on editable controls in VisualStylesMode.Net11 also change the border color.

After

Editable-control border color is independent from ForeColor.

Light mode:

Dark mode:

Test methodology

• Manually

Test environment(s)

• .net11.0.0-rc.1.26404.101

Microsoft Reviewers: Open in CodeFlow

[Copilot]

Pull request overview

This PR fixes a VisualStylesMode.Net11 rendering issue where changing ForeColor on editable controls unintentionally changes the control border color. It centralizes “editable text control” border-color selection in ModernControlColorMath and updates the NET11 renderers for TextBoxBase, ComboBox, and UpDownBase to use this shared logic (including a dark-mode-specific border color).

Changes:

• Added shared editable-control border color selection and enabled/disabled handling in ModernControlColorMath.
• Updated TextBoxBase and UpDownBase modern border rendering to use the shared border color instead of ForeColor.
• Updated ComboBox.ModernComboAdapter border rendering to use the shared border color (and simplified border-color selection logic).

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
src/System.Windows.Forms/System/Windows/Forms/Rendering/ModernControlColorMath.cs	Introduces shared border color defaults and an enabled/disabled border color helper.
src/System.Windows.Forms/System/Windows/Forms/Controls/UpDown/UpDownBase.cs	Uses shared border color logic for NET11 modern border rendering instead of ForeColor.
src/System.Windows.Forms/System/Windows/Forms/Controls/TextBox/TextBoxBase.cs	Uses shared border color logic for NET11 non-client painting instead of ForeColor.
src/System.Windows.Forms/System/Windows/Forms/Controls/ComboBox/ComboBox.ModernComboAdapter.cs	Switches modern ComboBox border rendering to shared border color logic and simplifies the helper signature.

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[ricardobossan]

This PR makes the same one-line change in three places, swapping the color source to ModernControlColorMath.TextControlBorderColor. In ComboBox.ModernComboAdapter.GetBorderColor (ComboBox.ModernComboAdapter.cs:412), that line sits inside a pre-existing if (!comboBox.Enabled) return ModernControlColorMath.GetDisabledBorderColor(); branch (not added by this PR, it was already there), so ComboBox picks up disabled-state handling for free:

private static Color GetBorderColor(ComboBox comboBox, bool useAccent)
{
    if (!comboBox.Enabled)
    {
        return ModernControlColorMath.GetDisabledBorderColor();
    }

    return useAccent
        ? Application.SystemVisualSettings.AccentColor
        : ModernControlColorMath.TextControlBorderColor;
}

TextBoxBase.cs:2668 and UpDownBase.cs:1089 never had an equivalent branch, so the same edit there just sets adornerColor unconditionally, with no Enabled check before or after. Since this PR is already touching the same color logic in all three places, is it worth adding the same disabled branch to TextBoxBase/UpDownBase here too, so they don't diverge from ComboBox?

[ricardobossan]

Please consider adding an XML doc like its siblings in this file?