Fix double newline in Println with trailing newlines

[veeceey]

Fixes #241

Problem

When using Println() or SprintlnFunc() with strings that already contain trailing newlines, the output would have double newlines:

c := color.New(color.FgRed)
c.Println("Hello\n")  // Outputs: "Hello\n\n" (double newline)

This happened because:

1. The input string has \n
2. fmt.Sprintln() adds another \n
3. TrimSuffix only removes the one added by Sprintln, leaving the original
4. Println adds yet another \n via fmt.Fprintln

Solution

Changed sprintln() helper to use TrimRight instead of TrimSuffix. This removes ALL trailing newlines from the formatted string, ensuring Println functions always add exactly one newline regardless of input.

Changes

• Modified sprintln() to use strings.TrimRight() to strip all trailing newlines
• Updated TestIssue218 to reflect the new normalized behavior
• Added TestTrailingNewline with comprehensive test cases

Testing

All existing tests pass. Manual verification:

c := color.New(color.FgRed)

// Before: double newline
// After: single newline
c.Println("Hello\n")

// SprintlnFunc also fixed
fn := c.SprintlnFunc()
result := fn("World\n")
// Before: "World\n\n"
// After: "World\n"

The behavior now matches user expectations - Println always produces exactly one trailing newline, making it more predictable when working with dynamic strings.

[veeceey]

Manual Test Results

Ran comprehensive tests to verify the fix works correctly:

=== Test 1: Println with string ending in \n ===
Hello
--- End of Test 1 ---

=== Test 2: Println with normal string ===
Hello
--- End of Test 2 ---

=== Test 3: SprintlnFunc with string ending in \n ===
Result: "World\n"
World
--- End of Test 3 ---

=== Test 4: SprintFunc with string ending in \n ===
Result: "Test\n"
Test
--- End of Test 4 ---

Test 1 and 3 show the fix working - no more double newlines. The output is now consistent regardless of whether the input has trailing newlines.

All package tests pass:

$ go test ./...
ok  	github.com/fatih/color	0.236s

[veeceey]

gentle ping — still hoping to get this one in. let me know if you'd like any changes

[veeceey]

any feedback on this? let me know!