Fixes Pausing and Unpausing in KIT GUI (#5178)

# Description

Previously, when pausing the Kit GUI, the viz markers would keep moving
regardless of pause state, and articulations would remain paused even
when the simulation was unpaused (played).

This fixes the kit gui visualization to respect the pause state
(targeting ``develop``).

Fixes #4279 (when this issue
was reported, on ``main``, the viz markers would be paused correctly,
but the articulations would remain paused after unpausing, hotfix for
``main`` here for reference:
https://gist.github.com/garylvov/9919d936d0393d548d51d7deaf473402)

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## Type of change

<!-- As you go through the list, delete the ones that are not
applicable. -->

- Bug fix (non-breaking change which fixes an issue)

## Screenshots

Tested with ``python scripts/reinforcement_learning/rsl_rl/train.py
--task Isaac-Velocity-Flat-Anymal-D-v0 --num_envs 4 --viz kit``


Before fix, on ``develop``

![pause_unpause_broken](https://github.com/user-attachments/assets/8b923fef-d528-48ea-811e-fc623fb62baa)

After fix, on ``develop``


![pause_unpause_develop_fixed](https://github.com/user-attachments/assets/044735a9-49c7-4279-a874-405ce15c5c30)

## Checklist

- [X] I have read and understood the [contribution
guidelines](https://isaac-sim.github.io/IsaacLab/main/source/refs/contributing.html)
- [X] I have run the [`pre-commit` checks](https://pre-commit.com/) with
`./isaaclab.sh --format`
- [X] I have made corresponding changes to the documentation
- [X] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [X] I have updated the changelog and the corresponding version in the
extension's `config/extension.toml` file
- [X] I have added my name to the `CONTRIBUTORS.md` or my name already
exists there

<!--

---------

Signed-off-by: garylvov <67614381+garylvov@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Co-authored-by: Antoine RICHARD <antoiner@nvidia.com>

Author: 3 people

source/isaaclab/config/extension.toml

@@ -1,7 +1,7 @@
 [package]
 
 # Note: Semantic Versioning is used: https://semver.org/
-version = "4.5.27"
+version = "4.5.28"
 
 # Description
 title = "Isaac Lab framework for Robot Learning"

source/isaaclab/docs/CHANGELOG.rst

@@ -1,6 +1,17 @@
 Changelog
 ---------
 
+4.5.28 (2026-04-10)
+~~~~~~~~~~~~~~~~~~~
+
+Added
+^^^^^
+
+* Added :meth:`~isaaclab.physics.PhysicsManager.wait_for_playing` hook and
+  integrated it into :meth:`~isaaclab.sim.SimulationContext.step` so the
+  training loop blocks while the Kit GUI timeline is paused.
+
+
 4.5.27 (2026-04-08)
 ~~~~~~~~~~~~~~~~~~~
 

source/isaaclab/isaaclab/physics/physics_manager.py

@@ -324,6 +324,11 @@ def stop(cls) -> None:
         """Stop physics simulation. Default is no-op."""
         pass
 
+    @classmethod
+    def wait_for_playing(cls) -> None:
+        """Block until the timeline is playing. Default is no-op."""
+        pass
+
     @classmethod
     def get_backend(cls) -> str:
         """Get the tensor backend being used ("numpy" or "torch")."""

source/isaaclab/isaaclab/sim/simulation_context.py

@@ -654,9 +654,15 @@ def reset(self, soft: bool = False) -> None:
     def step(self, render: bool = True) -> None:
         """Step physics and optionally render.
 
+        If the timeline is paused (e.g. via the GUI), this method blocks and keeps
+        the visualizer responsive until the timeline is resumed or stopped.
+
         Args:
             render: Whether to render the scene after stepping. Defaults to True.
         """
+        # Block while the GUI timeline is paused so the entire training loop freezes.
+        # See: https://github.com/isaac-sim/IsaacLab/issues/4279
+        self.physics_manager.wait_for_playing()
         self._physics_step_count += 1
         self.physics_manager.step()
         if render and self.is_rendering:

source/isaaclab_physx/config/extension.toml

@@ -1,7 +1,7 @@
 [package]
 
 # Note: Semantic Versioning is used: https://semver.org/
-version = "0.5.13"
+version = "0.5.14"
 
 # Description
 title = "PhysX simulation interfaces for IsaacLab core package"

source/isaaclab_physx/docs/CHANGELOG.rst

@@ -1,6 +1,20 @@
 Changelog
 ---------
 
+0.5.14 (2026-04-06)
+~~~~~~~~~~~~~~~~~~~
+
+Fixed
+^^^^^
+
+* Fixed the simulation training loop not pausing when the Kit GUI timeline is
+  paused. :meth:`~isaaclab_physx.physics.PhysxManager.wait_for_playing` now
+  blocks and keeps the GUI responsive until the timeline is resumed or stopped.
+* Fixed articulation visualization freezing after pausing and unpausing the
+  simulation through the headed GUI in Isaac Sim 5.1+. Articulation meshes now
+  remain visually updated after resuming.
+
+
 0.5.13 (2026-03-25)
 ~~~~~~~~~~~~~~~~~~~
 

source/isaaclab_physx/isaaclab_physx/physics/physx_manager.py

@@ -28,7 +28,7 @@
 import omni.physx
 import omni.timeline
 import omni.usd
-from pxr import Sdf
+from pxr import Sdf, UsdUtils
 
 import isaaclab.sim as sim_utils
 from isaaclab.physics import CallbackHandle, PhysicsEvent, PhysicsManager
@@ -290,6 +290,30 @@ def stop(cls) -> None:
         # Pump events so timeline callbacks fire synchronously
         omni.kit.app.get_app().update()
 
+    @classmethod
+    def wait_for_playing(cls) -> None:
+        """Block until the timeline is playing, keeping the GUI responsive.
+
+        After resume, forces a fabric re-sync so articulation meshes unfreeze.
+        See: https://github.com/isaac-sim/IsaacLab/issues/4279
+        """
+        if cls._timeline.is_playing():
+            return
+        app = omni.kit.app.get_app()
+        while not cls._timeline.is_playing():
+            app.update()
+            if cls._timeline.is_stopped():
+                break
+        # Force fabric to re-sync articulation transforms after resume.
+        # detach/attach resets the FabricManager, then we immediately push
+        # current poses so the first render after resume shows correct state.
+        if not cls._timeline.is_stopped():
+            cls._re_sync_fabric()
+            if cls._view is not None:
+                cls._view.update_articulations_kinematic()
+            if cls._update_fabric is not None:
+                cls._update_fabric(0.0, 0.0)
+
     @classmethod
     def close(cls) -> None:
         """Clean up physics resources."""
@@ -585,6 +609,42 @@ def _load_fabric(cls) -> None:
         sim.set_setting("/isaaclab/fabric_enabled", use_fabric)  # type: ignore[union-attr]
         sim.set_setting("/physics/visualizationDisplaySimulationOutput", False)  # type: ignore[union-attr]
 
+    @classmethod
+    def _re_sync_fabric(cls) -> None:
+        """Force the PhysX fabric extension to re-synchronize after a pause/resume transition.
+
+        Starting with PhysX fabric 107.3.21 (Isaac Sim 5.1), the FabricManager skips writing
+        initial articulation poses to fabric on subsequent resumes, causing articulation meshes
+        to freeze visually while physics continues to run.
+
+        The workaround detaches and re-attaches the USD stage on the fabric interface, forcing
+        the FabricManager to fully reinitialize and write transforms into fabric so that Hydra
+        picks them up.
+        """
+        if cls._fabric is None:
+            return
+        sim = PhysicsManager._sim
+        if sim is None:
+            return
+        stage = sim.stage
+        if stage is None:
+            return
+        stage_id = UsdUtils.StageCache.Get().GetId(stage).ToLongInt()
+        if stage_id <= 0:
+            return
+        try:
+            cls._fabric.detach_stage()
+        except Exception:
+            logger.warning("Failed to detach fabric stage during re-sync. Articulation visuals may be stale.")
+            return
+        try:
+            cls._fabric.attach_stage(stage_id)
+        except Exception:
+            logger.error(
+                "Could not re-attach fabric stage. Articulation visuals will be broken until next reset.",
+                exc_info=True,
+            )
+
     @classmethod
     def _warmup_and_create_views(cls) -> None:
         """Warm-start physics and create simulation views."""