CTabFolder: Improve readability, performance, simplification, and test coverage

[vogella]

Overview

CTabFolder (4,257 lines), CTabFolderRenderer (1,338 lines), CTabItem (540 lines), and CTabFolderLayout (123 lines) have significant opportunities for improvement in readability, performance, code simplification, and test coverage.

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1. Readability Improvements

• Extract and simplify updateItems() (CTabFolder): 83-line method with 4 near-identical loops computing tab widths. Extract into helper methods and add comments explaining the priority-based visibility algorithm.
• Split drawSelected() (CTabFolderRenderer): 187-line method with deep nesting and shape array index arithmetic. Split into shape construction, highlight drawing, and text rendering.
• Split computeControlBounds() (CTabFolder): 182-line method mixing layout, positioning, and wrapping. Split into leading/trailing control computation.
• Replace magic numbers with named constants: e.g. 16 for button width (CTabFolder), unnamed margins/spacing in CTabFolderRenderer.
• Fix misleading field initialization in CTabItem: int x,y,width,height = 0; — only height is explicitly initialized (others default to 0 by JLS but this is misleading).

2. Performance Improvements

• Replace array copying in addTabControl()/removeTabControl() (CTabFolder): 4 parallel arrays are fully copied on each add/remove. Use ArrayList or grow-by-factor strategy.
• Cache renderer.computeSize() results in updateItems(): Called 4+ times per item in nested loops. Cache results to avoid redundant computation.
• Avoid Rectangle allocation on every mouse move (CTabFolder onMouseMove()): Reuse a field instead of new Rectangle() in hot path.
• Reduce Region allocation in drawBackground() (CTabFolderRenderer): Creates and disposes Region objects on every call.
• Cache chevron image in updateChevronImage(): Currently disposes and recreates on every call.

3. Code Simplification

• Extract duplicate min/max button logic in updateButtons() (CTabFolder): Near-identical blocks for min/max button creation. Extract to shared helper method.
• Unify close button positioning in setItemLocation() (CTabFolder): Separate code paths for single/multi mode share most logic — consolidate.
• Consolidate drawBackground() overloads (CTabFolderRenderer): 3 overloads with nearly identical logic.
• Extract repeated null-coalescing pattern in CTabItem: getForeground(), getSelectionForeground(), getFont() are identical patterns.
• Simplify setSelectionBackground() (CTabFolder): 81 lines, 66 of which are validation/comparison. Extract colorsEqual() helper.
• Remove or document commented-out code (CTabItem) and consider timeline for deprecated field removal.

[vogella]

@HeikoKlare feel free to point your AI agent to work on any of these issues. I will for now move away from this topic.

[vogella]

Test coverage increase: #3222 I tried to include only reasonable tests).