fix(tui): keep copy label background under "copied" flash

[dgageot]

When a user clicks a copy button, the label briefly swaps to a "copied" confirmation. For code blocks, the background of that flash was already forced to match the code block's band. For user-message copy labels, which sit on the BackgroundAlt band, no such fix existed, so the transient label rendered with a transparent background and punched a visible hole in the colored band.

The fix introduces backgroundAt(line, col), which walks the ANSI SGR escape sequences of a rendered line and returns the background color in effect at the given column — handling truecolor, 256-indexed, basic, and bright colors, plus resets. The "copied" flash now calls this helper for all copy labels, replacing the code-block-only special case with a single, general path.

New regression tests cover the user-message band case, the code-block band case, and a table-driven suite for backgroundAt itself.

[docker-agent]

Assessment: 🔴 CRITICAL

1 high-severity finding in new code.

[docker-agent]

[high] backgroundAt can spin in an infinite loop when ansi.DecodeSequence returns n == 0

ansi.DecodeSequence can return n == 0 when the first byte of the remaining input is an invalid UTF-8 sequence (e.g., an isolated continuation byte 0x80–0xBF). In that case:

• line = line[n:] is a no-op — line does not shrink
• line != "" remains true
• w never advances (the invalid byte has visual width == 0)
• The loop condition w <= col stays satisfied

This causes a busy infinite loop, freezing the TUI.

From charmbracelet/x/ansi@v0.11.7/parser_decode.go, the "Invalid UTF-8 sequence" return path is:

// Invalid UTF-8 sequence
return b[:i], 0, i, NormalState  // i == 0 on first byte → n == 0

This path is reachable whenever a rendered line contains an isolated continuation byte or other invalid UTF-8. While charmbracelet's own rendering is unlikely to emit such bytes, backgroundAt is called on arbitrary terminal lines, so robustness matters.

Fix: add an early-out guard immediately after the DecodeSequence call:

Suggested change

	seq, width, n, newState := ansi.DecodeSequence(line, state, p)
	seq, width, n, newState := ansi.DecodeSequence(line, state, p)
	if n == 0 {
	break
	}

Confidence	Score
🟡 moderate	57/100

[docker-agent]

Assessment: 🟢 APPROVE