Research continuous SplitContainer repainting and derive the implementation approach

[KlausLoeffelmann]

Objective

Research the SplitContainer splitter-movement, layout, invalidation, and painting paths and derive a concrete implementation approach for continuous Panel1 and Panel2 repainting in VisualStylesMode.Net11 and later.

Work

• Reproduce and characterize the failure with representative content in both panels.
• Trace mouse and keyboard splitter movement through SplitContainer, Splitter, layout, invalidation, buffering, and paint paths.
• Compare Classic mode with VisualStylesMode.Net11 and later to identify the behavioral divergence.
• Determine whether the defect is caused by invalidation scope/timing, layout suspension, buffering, native window behavior, rendering-mode logic, or a combination.
• Review existing WinForms patterns and tests before introducing new helpers or behavior.
• Evaluate candidate approaches for correctness, flicker, re-entrancy, layout frequency, CPU cost, handle churn, and compatibility.
• Identify the narrowest mode-gated implementation that keeps both panels visually current.
• Define the code-generation/prompting approach if Copilot is used: relevant files and symbols, invariants, prohibited shortcuts, tests to create first, and validation requirements.

Deliverables

• A concise root-cause analysis.
• Recommended implementation approach and rejected alternatives with rationale.
• Expected files and code paths to change.
• Risks and mitigations, including accessibility, performance, flicker, layout recursion, and compatibility.
• A test plan that the implementation and test tasks can follow.

Completion criteria

• The issue reproduces reliably.
• The relevant call paths and mode-specific divergence are documented.
• The proposed approach explains why both panels repaint continuously.
• Performance, accessibility, and compatibility risks are addressed.
• The implementation or code-generation prompt is specific enough to execute without architectural guesswork.