Fix: script-default visualizers not cleared in headless mode (--headless and HEADLESS=1)

[ChaitanyaParate]

Summary

Fixes a runtime crash in AppLauncher when a task script registers a default visualizer via parser.set_defaults(visualizer=["kit"]) and the process is launched in headless mode.

Fixes #7403

Root cause

_resolve_visualizer_settings incorrectly promoted set_defaults() values to explicit user intent (_cli_visualizer_explicit = True), even though the user never typed --viz on the command line.

Fix

• Instead of unconditionally clearing visualizers (which conflicts with develop's newly supported HEADLESS=1 + Kit visualizer livestreaming lifecycle), this PR fixes the root cause directly by tracking whether the arguments came from set_defaults.
• Added parser.set_defaults(visualizer_explicit=False) in add_app_launcher_args.
• Updated _resolve_visualizer_settings to correctly respect visualizer_explicit=False while preserving the presence-based fallback for programmatic kwargs (e.g., AppLauncher(visualizer=["kit"])).

Testing

• Added test_visualizer_argparse_set_defaults_is_not_explicit and test_visualizer_direct_kwargs_is_explicit to verify intent resolution logic.
• Updated test_matrix_headless_with_viz_names_takes_precedence to align with the new develop lifecycle (expecting visualizers not to be unconditionally cleared under headless modes).

Checklist

• Pre-commit hooks run and passed (./isaaclab.sh -f)
• Tests added / updated
• CHANGELOG.rst updated (via changelog fragment)

[greptile-apps]

Greptile Summary

This PR distinguishes script-defined argparse visualizer defaults from explicit --visualizer input so headless launches do not incorrectly enforce CLI visualizer behavior.

• Adds a non-explicit default marker when registering launcher arguments.
• Preserves presence-based intent detection for direct programmatic launcher kwargs.
• Adds coverage for argparse defaults, direct kwargs, aliases, and headless visualizer resolution.
• Documents the corrected headless-launch behavior.

Confidence Score: 5/5

The PR appears safe to merge, with no concrete blocking or independently actionable non-blocking defects identified.

The new intent marker correctly separates parser defaults from explicit CLI input while retaining direct-kwargs detection, and downstream handling prevents non-explicit defaults from reaching the explicit-visualizer error path.

Important Files Changed

Filename	Overview
source/isaaclab/isaaclab/app/app_launcher.py	Introduces an argparse intent marker and updates visualizer resolution to distinguish parser defaults from explicit CLI or direct-kwargs selections.
source/isaaclab/test/app/test_kwarg_launch.py	Updates alias expectations and adds focused tests for visualizer intent and headless behavior.
source/isaaclab/changelog.d/chaitanya-fix-headless-visualizer-resolution.rst	Accurately documents the corrected handling of script-defined visualizer defaults.

_{Reviews (1): Last reviewed commit: "Fix headless visualizer bug related to p..." | Re-trigger Greptile}

[isaaclab-review-bot]

Isaac Lab Review Bot

The explicitness sentinel correctly distinguishes argparse defaults from explicit --viz input and direct kwargs, but it introduces a regression: a script-defined default such as visualizer=["kit"] no longer contributes to headless intent, so an ordinary launch is forced headless when visualizer_intent is absent.

• Design and architecture: The tri-state explicitness handling is appropriate, but script-defined visualizer defaults are demoted without being transferred to the existing configuration-intent path. Their selected backend remains recorded while headless resolution behaves as though no visualizer was selected.
• API: Explicit --viz and AppLauncher(visualizer=[...]) retain their existing explicit-selection behavior. However, the argparse integration surface changes materially: parser.set_defaults(visualizer=["kit"]) now causes a normal launch to resolve as headless unless the script separately supplies visualizer_intent.
• Implementation: Because visualizer_explicit=False bypasses the presence fallback, _resolve_headless_settings reaches its non-explicit branch and consults only _cfg_has_any_visualizers and _cfg_has_kit_visualizer. With no visualizer_intent, both are false, forcing headless even though _cli_visualizer_types contains kit. Non-explicit argparse defaults should feed the configuration-intent state or otherwise preserve their normal-launch semantics.

Minor fixes needed. Posted 1 actionable finding inline.

Automated review; human maintainers own approval decisions.

[isaaclab-review-bot]

🟡 Warning · Design Architecture — Script default visualizers now silently force headless

With visualizer_explicit=False always present in the parsed namespace, a script using parser.set_defaults(visualizer=["kit"]) takes the else branch of _resolve_headless_settings (lines 938-954). That branch consults only visualizer_intent, which such scripts do not set, so _headless becomes True even on a plain non-headless run, while _cli_visualizer_types still holds kit. Map non-explicit argparse visualizer defaults into _cfg_has_any_visualizers/_cfg_has_kit_visualizer instead of discarding them.

[ChaitanyaParate]

non-explicit argparse visualizer defaults are now properly mapped into _cfg_has_any_visualizers and _cfg_has_kit_visualizer