Fix Issue 14005: [DarkMode] the Pane in the Anchor Editor should adapt to the dark mode

[SimonZhao888]

Fixes #14005

Proposed changes

Updated the center placeholder (ControlPlaceholder) visual rendering in AnchorEditor.AnchorUI for dark mode.

• In dark mode:
  • Set center placeholder background to SystemColors.ControlDark.
  • Replaced the previous button-style painting with explicit dark fill + border drawing (FillRectangle + DrawRectangle) to avoid the bright/white center block.
• In non-dark mode:
  • Kept existing rendering behavior (ControlPaint.DrawButton(..., ButtonState.Normal)) unchanged.

Customer Impact

• Updated the visual rendering of the central placeholder (ControlPlaceholder) in AnchorEditor.AnchorUI to ensure compatibility with dark mode.

Regression?

• No

Risk

• Mini

Screenshots

Before

After

Dark Mode

Normal

Test methodology

• Manually

Test environment(s)

• 11.0.0-preview.6.26317

Microsoft Reviewers: Open in CodeFlow

[Copilot]

Pull request overview

Updates the AnchorEditor drop-down UI so the center placeholder renders appropriately under WinForms dark mode, avoiding the overly bright/white center block reported in #14005.

Changes:

• Set the center placeholder (ControlPlaceholder) background to SystemColors.ControlDark when Application.IsDarkModeEnabled.
• In dark mode, replace ControlPaint.DrawButton(...) with explicit fill + border drawing; keep the existing button-style rendering for non-dark mode.

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[LeafShi1]

Minor: the dark-mode decision is duplicated here (for BackColor) and again in OnPaint. Since BackColor is captured only at construction while OnPaint re-evaluates Application.IsDarkModeEnabled on every paint, the two could diverge if the process color mode changes after this control is created. Consider centralizing the decision, or relying solely on OnPaint.

[SimonZhao888]

ControlPaint.DrawButton(e.Graphics, rc, ButtonState.Normal); draws a button but does not fill it with color; therefore, we need to fill it when in dark mode.

[LeafShi1]

BackColor is already set to SystemColors.ControlDark in the constructor, so the default OnPaintBackground fills the client area with that same color before OnPaint runs. This explicit FillRectangle repaints the identical color and appears redundant — the dark branch really only needs the DrawRectangle border. Consider dropping the fill.

[SimonZhao888]

In reality, it does not perform automatic filling; we still need to fill it in manually.

[LeafShi1]

In the dark palette SystemColors.WindowFrame is typically near-black, so a border drawn with it over a ControlDark fill against the surrounding dark container may have very low contrast. Could you confirm from the dark-mode screenshot that this border is actually visible? If it isn't, a lighter system color (e.g. SystemColors.ControlLight or ActiveBorder) would read better and stay consistent with the 3D button edge shown in light mode.

[SimonZhao888]

Adding a border is not recommended, as borders are typically used to indicate states like selection or clicking, whereas a ControlPlaceholder cannot be selected.