Fix DataGridView ComboBox and Button cell rendering in dark mode

[LeafShi1]

Fixes #15077

Root Cause

DataGridViewComboBoxCell continued using visual-style colors that could return dark text on a dark background.
DataGridViewButtonCell used the legacy visual-style renderer instead of the dark-mode renderers used by regular WinForms buttons.

Proposed changes

• Render DataGridView ComboBox text, border, and drop-down button using dark-compatible colors.
• Reuse the existing WinForms dark-mode Button renderers for DataGridView Button cells.

Customer Impact

• Customers using dark-mode DataGridViews receive readable ComboBox content and consistent Button rendering, with clearer mouse and keyboard interaction feedback.

Regression?

• No

Risk

• minimal

Screenshots

Before

ComboBox cell text could be unreadable against a dark background.
DataGridView Button cells did not visually match regular WinForms buttons in dark mode.

After

ComboBox text is readable in both selected and unselected dark-mode cells.
DataGridView Button cells use the same dark-mode rendering as regular WinForms buttons.

AfterChanges.mp4

Test methodology

• Manual test and unit test

Test environment(s)

• .net 11.0.0-beta.26458.117

Microsoft Reviewers: Open in CodeFlow

[Copilot]

🟡 Changes recommended

The current dark-mode rendering logic introduces correctness issues (hard-coded colors and state-dependent text color mismatches) that should be addressed before merging.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

Improves WinForms DataGridView dark-mode theming by adjusting how ComboBox and Button cells choose colors/renderers so text remains readable and button cells visually match standard dark-mode buttons.

Changes:

• Update DataGridViewComboBoxCell rendering to use dark-mode-compatible text/drop-down button visuals.
• Update DataGridViewButtonCell rendering to reuse existing WinForms dark-mode button renderers and surface EffectiveVisualStylesMode internally for renderer selection.
• Add unit tests for the new internal helper logic.

File summaries

File	Description
src/test/unit/System.Windows.Forms/System/Windows/Forms/DataGridViewComboBoxCellTests.cs	Adds a unit test covering ComboBox cell text color selection under dark-mode conditions.
src/test/unit/System.Windows.Forms/System/Windows/Forms/DataGridViewButtonCellTests.cs	Adds a unit test for Button cell style color selection helper.
src/System.Windows.Forms/System/Windows/Forms/Controls/DataGridView/DataGridViewComboBoxCell.DataGridViewComboBoxCellRenderer.cs	Adjusts ComboBox drop-down button rendering behavior for dark mode.
src/System.Windows.Forms/System/Windows/Forms/Controls/DataGridView/DataGridViewComboBoxCell.cs	Refactors ComboBox text color selection into a helper with dark-mode branching.
src/System.Windows.Forms/System/Windows/Forms/Controls/DataGridView/DataGridViewButtonCell.DataGridViewButtonCellRenderer.cs	Introduces dark-mode button renderers and returns content bounds + text color from the draw routine.
src/System.Windows.Forms/System/Windows/Forms/Controls/DataGridView/DataGridViewButtonCell.cs	Integrates dark-mode renderer usage into Button cell painting and extracts style color selection helper.
src/System.Windows.Forms/System/Windows/Forms/Controls/DataGridView/DataGridView.cs	Exposes an internal accessor for effective visual styles mode for internal consumers.

Review details

• Files reviewed: 7/7 changed files
• Comments generated: 4
• Review effort level: Lite

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 37.24166%. Comparing base (8efd9a9) to head (819543a).

Additional details and impacted files

@@              Coverage Diff              @@
##                main      #15075   +/-   ##
=============================================
  Coverage   37.24166%   37.24166%           
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  Files            246         246           
  Lines           9774        9774           
  Branches        1029        1029           
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  Hits            3640        3640           
  Misses          5970        5970           
  Partials         164         164           

Flag	Coverage Δ
Debug	37.24166% <ø> (ø)
production	39.36526% <ø> (ø)
test	20.64923% <ø> (ø)
unit	39.36526% <ø> (ø)

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[Copilot]

🟡 Changes recommended

The dark-mode ComboBox drop-down button rendering path appears to ignore RTL handling, which risks a functional rendering regression for right-to-left DataGridViews.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Review details

• Files reviewed: 7/7 changed files
• Comments generated: 2
• Review effort level: Lite