Validate mutually exclusive CLI flags

[Rupam0710]

Search before asking

• I searched in the issues and found nothing similar.

Motivation

Users can currently combine incompatible CLI flags without receiving feedback, leading to confusion:

• --fullscreen with window sizing options (--width, --height, --min-width, --min-height) - conflicting approaches to fill the window
• --fullscreen with --maximize - two different window fill modes
• --start-to-tray with --maximize - can't maximize a window that starts hidden

These combinations are silently ignored or produce unexpected behavior. Early validation with clear error messages would improve user experience.

Solution

Implement validateMutuallyExclusiveFlags() validation function that:

• Checks for all 6 conflict scenarios immediately after config loading (input phase)
• Provides clear error messages with helpful hints for remediation
• Returns proper exit code (2) for invalid input
• Works seamlessly with --json machine mode
• Follows the same pattern as recent validation PRs (fix: report bad --zoom and --hide-on-close values as invalid input #1381, fix: reject fractional zoom in config files #1380)

Implementation includes:

• New validation function in bin/utils/validate.ts
• Integration in bin/cli.ts after config merge
• 23 comprehensive test cases covering all scenarios

Alternatives

1. Silent ignore with warning - Considered warning users but still ignoring the conflicting flag. Rejected because it's less clear than hard validation.

2. Config-file only validation - Could validate only in config files, not CLI. Rejected because CLI users deserve the same protection.

3. Runtime error - Could let the app start and error at runtime. Rejected because early validation in input phase is clearer and prevents wasted build time.

Anything else?

This follows the established pattern from recent merged PRs:

• PR fix: report bad --zoom and --hide-on-close values as invalid input #1381: "report bad --zoom and --hide-on-close values"
• PR fix: reject fractional zoom in config files #1380: "reject fractional zoom in config files"

Test coverage: 23 new tests + 516 existing tests all passing (519 total, 0 regressions)
Exit codes and error format are compatible with --json machine mode.

Are you willing to submit a PR?

• I'm willing to submit a PR!