diff --git a/DeepDrftPublic.Client/Controls/WaveformVisualizerControls.razor b/DeepDrftPublic.Client/Controls/WaveformVisualizerControls.razor
index e8a6285..731ef36 100644
--- a/DeepDrftPublic.Client/Controls/WaveformVisualizerControls.razor
+++ b/DeepDrftPublic.Client/Controls/WaveformVisualizerControls.razor
@@ -226,12 +226,14 @@
     private void ToggleLava()
     {
         ControlState.LavaEnabled = !ControlState.LavaEnabled;
+        ControlState.CoerceTheaterMode();
         ControlState.NotifyChanged();
     }
 
     private void ToggleWaveform()
     {
         ControlState.WaveformEnabled = !ControlState.WaveformEnabled;
+        ControlState.CoerceTheaterMode();
         ControlState.NotifyChanged();
     }
 
diff --git a/DeepDrftPublic.Client/Services/WaveformVisualizerControlState.cs b/DeepDrftPublic.Client/Services/WaveformVisualizerControlState.cs
index 556bf98..c20faa1 100644
--- a/DeepDrftPublic.Client/Services/WaveformVisualizerControlState.cs
+++ b/DeepDrftPublic.Client/Services/WaveformVisualizerControlState.cs
@@ -160,6 +160,19 @@ public sealed class WaveformVisualizerControlState
     /// </summary>
     public event Action? Changed;
 
+    /// <summary>
+    /// Enforces the Theater-Mode invariant: Theater Mode cannot remain on when both visualizer
+    /// subsystems are off (there is nothing to go to theater FOR). Call this after mutating
+    /// <see cref="LavaEnabled"/> or <see cref="WaveformEnabled"/> and before
+    /// <see cref="NotifyChanged"/> so all observers see a consistent, coerced state in the same
+    /// <see cref="Changed"/> cycle.
+    /// </summary>
+    public void CoerceTheaterMode()
+    {
+        if (TheaterMode && !LavaEnabled && !WaveformEnabled)
+            TheaterMode = false;
+    }
+
     /// <summary>Raise <see cref="Changed"/>. Called by the controls component after mutating a value.</summary>
     public void NotifyChanged() => Changed?.Invoke();
 }
diff --git a/DeepDrftTests/WaveformVisualizerControlStateTests.cs b/DeepDrftTests/WaveformVisualizerControlStateTests.cs
new file mode 100644
index 0000000..42c0664
--- /dev/null
+++ b/DeepDrftTests/WaveformVisualizerControlStateTests.cs
@@ -0,0 +1,91 @@
+using DeepDrftPublic.Client.Services;
+
+namespace DeepDrftTests;
+
+/// <summary>
+/// Unit tests for the Theater-Mode auto-exit invariant on <see cref="WaveformVisualizerControlState"/>
+/// (Phase 20 bug fix): when both subsystems are disabled, <see cref="WaveformVisualizerControlState.CoerceTheaterMode"/>
+/// must force <c>TheaterMode = false</c> so observers never see a stranded-theater state.
+/// </summary>
+[TestFixture]
+public class WaveformVisualizerControlStateTests
+{
+    private WaveformVisualizerControlState _state = null!;
+
+    [SetUp]
+    public void SetUp() => _state = new WaveformVisualizerControlState();
+
+    // ── CoerceTheaterMode guard ──
+
+    // Both off + Theater on → coerce exits theater.
+    [Test]
+    public void CoerceTheaterMode_BothOff_TheaterBecomesfalse()
+    {
+        _state.TheaterMode = true;
+        _state.LavaEnabled = false;
+        _state.WaveformEnabled = false;
+
+        _state.CoerceTheaterMode();
+
+        Assert.That(_state.TheaterMode, Is.False);
+    }
+
+    // Lava still on → theater is left alone even if waveform is off.
+    [Test]
+    public void CoerceTheaterMode_LavaOnWaveformOff_TheaterPreserved()
+    {
+        _state.TheaterMode = true;
+        _state.LavaEnabled = true;
+        _state.WaveformEnabled = false;
+
+        _state.CoerceTheaterMode();
+
+        Assert.That(_state.TheaterMode, Is.True);
+    }
+
+    // Waveform still on → theater is left alone even if lava is off.
+    [Test]
+    public void CoerceTheaterMode_WaveformOnLavaOff_TheaterPreserved()
+    {
+        _state.TheaterMode = true;
+        _state.LavaEnabled = false;
+        _state.WaveformEnabled = true;
+
+        _state.CoerceTheaterMode();
+
+        Assert.That(_state.TheaterMode, Is.True);
+    }
+
+    // Theater already false + both off → no change (no false-positive write).
+    [Test]
+    public void CoerceTheaterMode_TheaterAlreadyFalse_NoChange()
+    {
+        _state.TheaterMode = false;
+        _state.LavaEnabled = false;
+        _state.WaveformEnabled = false;
+
+        _state.CoerceTheaterMode();
+
+        Assert.That(_state.TheaterMode, Is.False);
+    }
+
+    // ── Changed event fires once with coerced state visible ──
+
+    // Verify that after coercion, the Changed notification carries the already-corrected TheaterMode
+    // value — all observers see a consistent state in the single Changed cycle.
+    [Test]
+    public void NotifyChanged_AfterCoerce_ObserverSeesTheaterFalse()
+    {
+        _state.TheaterMode = true;
+        _state.LavaEnabled = false;
+        _state.WaveformEnabled = false;
+
+        bool? observedTheaterMode = null;
+        _state.Changed += () => observedTheaterMode = _state.TheaterMode;
+
+        _state.CoerceTheaterMode();
+        _state.NotifyChanged();
+
+        Assert.That(observedTheaterMode, Is.False);
+    }
+}