Pane viewer: render tmux colours from capture-pane -e #151

Summary

The MCP Apps pane viewer added in #149 draws a pane as plain text. Programs that colour their output (compilers, test runners, ls, prompts) lose that signal in the viewer, though tmux can supply it: capture-pane -e keeps the colour and style codes.

Motivation

Colour carries meaning on a terminal: red failures, green passes, highlighted matches. A viewer meant to show what is on the screen drops it.

Evidence

tmux does not replay the program's original bytes. It re-emits SGR codes from its grid, one code per attribute, with 39/49/0 resets, and it writes attribute codes of 10 or more as d:d (grid.c at 3.8):

		{ GRID_ATTR_UNDERSCORE_2, 42 },
		...
		{ GRID_ATTR_OVERLINE, 53 },
		...
			if (s[i] < 10)
				xsnprintf(tmp, sizeof tmp, "%d", s[i]);
			else {
				xsnprintf(tmp, sizeof tmp, "%d:%d", s[i] / 10,
				    s[i] % 10);

That yields 4:2 to 4:5 for underline styles, and 5:3 for overline (SGR 53). A parser that reads 5:3 as SGR 5 with a sub-parameter shows blink instead. Observed with tmux 3.8a, after a pane printed printf '\033[4:3mcurly\033[0m \033[53mover\033[0m \033[4:2mdbl\033[0m\n':

$ tmux capture-pane -p -e | cat -v

^[[4:3mcurly^[[0m ^[[5:3mover^[[0m ^[[4:2mdbl^[[0m

38/48/58 take ;5;n or ;2;r;g;b, always ;-separated, and OSC 8 hyperlinks may appear as \033]8;…. libtmux already exposes the flag as Pane.capture_pane(escape_sequences=True).

Proposal

Render the codes in the viewer page: a small SGR parser that turns each captured row into styled runs, placing the cursor by visible column. A local spike did this. The parser kept the visible text identical to the plain capture, handled the d:d forms, 256-colour and truecolor, and rendered a coloured pane in a browser host built on the ext-apps AppBridge.

How the page gets the codes is the open decision:

• An escape_sequences: bool = False option on snapshot_pane, which the page sets on its refresh calls. This is what the spike did. It adds 170 bytes to snapshot_pane's schema (6010 to 6180 in the tool-surface snapshot), paid by every client, for a feature only the viewer uses.
• app.readServerResource() on a tmux:// template that captures with -e. This costs no tool schema bytes. It needs the host's serverResources capability, and hosts without it fall back to plain text.

Either way, the model's own snapshot_pane call arrives without codes, so the page should fetch a coloured frame as soon as it renders the first result.

References

• mcp(feat): Confirm kill_session, add pane viewer and icons #149 adds the viewer (ui://libtmux-mcp/pane-viewer.html)
• tmux grid.c attribute table and d:d emission at 3.8
• MCP Apps extension