Refactor stickyScrollWidget.ts for improved code quality and readability

[Copilot]

This PR refactors the stickyScrollWidget.ts file to improve code quality, readability, and maintainability without changing any functionality or behavior.

Key Improvements

🔧 Constants Extraction

• Extracted magic numbers and strings into well-named constants:
  • FOLDING_ICON_WIDTH (26px)
  • Z_INDEX_LAST_LINE and Z_INDEX_REGULAR_LINE for z-index values
  • FOLDING_OPACITY_TRANSITION_DURATION (0.5s)
  • STICKY_WIDGET_ID for widget identifier
  • CSS class name constants (STICKY_LINE_CONTENT_CLASS, etc.)

📦 Method Decomposition

Broke down large, complex methods into focused, single-responsibility helpers:

_renderRootNode → smaller methods:

• _processExistingLines - handles existing sticky line updates
• _renderNewLines - renders new sticky lines from scratch
• _finalizeStickyLines - finalizes widget state and layout

_renderChildNode → smaller methods:

• _getActualInlineDecorations - safely extracts inline decorations
• _renderLineContent - handles line content rendering
• _createLineElement - creates line DOM elements
• _createLineNumberElement - creates line number DOM elements

🛠️ Utility Methods Added

• _shouldSkipStateUpdate - simplifies complex boolean logic in setState
• _setElementPosition - consolidates position styling patterns
• _setLineHeightAndHeight - consolidates height styling patterns
• _isValidLineIndex - provides consistent range checking

✨ Code Quality Enhancements

• Simplified complex conditional expressions for better readability
• Replaced traditional for loops with modern for-of syntax where appropriate
• Improved early returns in methods to reduce nesting
• Added JSDoc comments to key public methods following VS Code guidelines
• Better method organization and logical grouping
• Consistent use of extracted constants throughout

Before/After Example

Before:

const currentStateAndPreviousStateUndefined = !this._state && !state;
const currentStateDefinedAndEqualsPreviousState = this._state && this._state.equals(state);
if (currentStateAndPreviousStateUndefined || currentStateDefinedAndEqualsPreviousState) {
    return;
}

After:

if (this._shouldSkipStateUpdate(state)) {
    return;
}

private _shouldSkipStateUpdate(state: StickyScrollWidgetState | undefined): boolean {
    const bothStatesUndefined = !this._state && !state;
    const statesAreEqual = this._state && this._state.equals(state);
    return bothStatesUndefined || statesAreEqual;
}

All changes preserve existing functionality and public interfaces while significantly improving code readability and maintainability. The refactoring follows VS Code's coding guidelines including proper indentation, naming conventions, and JSDoc style comments.

Fixes #252331.

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