Fix #endregion indentation when followed by labels

[Copilot]

Fix #endregion indentation when followed by labels

Plan

• Reproduce the issue with a test case that demonstrates the bug
• Analyze the code to understand why #endregion is incorrectly affected by following labels
• Fix the indentation logic in CSharpTriviaFormatter to use GetMatchingDirective for #endregion
• Verify the test passes with the fix
• Run FormattingEngineTriviaTests to ensure no regressions (72/72 passed)
• Run broader formatting tests to ensure no regressions (652/652 passed)
• Security scan completed - no issues found
• Fix WorkItem URL to reference correct issue (#endregion directive improperly indented when followed by a label #75919)
• Simplify logic to always align #endregion with #region

Issue

When a #endregion directive is followed by a label, the formatter incorrectly indents the #endregion to the same level as the label, instead of matching the indentation of the corresponding #region directive.

Root Cause

The formatter was using "default indentation" for #region and #endregion directives. This default indentation is calculated based on the position where the directive appears, which can be influenced by the following token. When a label follows #endregion, the formatter's indentation calculation is affected by the label's reduced indentation, causing #endregion to be incorrectly indented.

Solution

Modified CSharpTriviaFormatter.GetLineColumnRuleBetween() to:

1. For #endregion directives, find the matching #region using GetMatchingDirective()
2. Always align #endregion with its matching #region indentation

This ensures #endregion always aligns with its matching #region, preventing labels or other following constructs from incorrectly influencing the indentation. This provides consistent and predictable behavior.

Testing

• Added test case EndRegionFollowedByLabel demonstrating the fix ✅
• Updated Preprocessor8 test to reflect the new consistent behavior ✅
• All 72 FormattingEngineTriviaTests pass ✅
• No security vulnerabilities detected ✅

Fixes #75919

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[CyrusNajmabadi]

@copilot all these urls should be #75919

[Copilot]

Fixed in a48ed5b

[CyrusNajmabadi]

@copilot i don't understand why we need to use this 'maximum' concept? i think it would suffice to always align the #endregion with the #region.

if there is a case that makes you think your logic is correct, then please add a test demonstrating why that's teh desirable behavior. Otherwise, simplify this.