Persist HUD layout on drag-end instead of only on teardown

HUD panel positions and brush values were written only from
OnApplicationQuit/OnDestroy, which is not guaranteed in a build (Alt-F4,
process kill, crash), so layout work was lost. DraggablePanel now exposes
an IEndDragHandler callback that saves the layout the moment a panel is
dropped. Brush-value changes are detected against a disk baseline in
LateUpdate and flushed after a 0.5s debounce so per-frame slider drags
do not write every frame. The teardown save path is removed.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Assets/Scripts/DraggablePanel.cs

@@ -14,7 +14,7 @@ using UnityEngine.EventSystems;
 /// corners in its parent, not from a fixed top-left assumption).
 /// </summary>
 [RequireComponent(typeof(RectTransform))]
-public class DraggablePanel : MonoBehaviour, IPointerDownHandler, IBeginDragHandler, IDragHandler
+public class DraggablePanel : MonoBehaviour, IPointerDownHandler, IBeginDragHandler, IDragHandler, IEndDragHandler
 {
     [Tooltip("Panel moved by the drag. Empty = this object's RectTransform.")]
     public RectTransform target;
@@ -22,6 +22,10 @@ public class DraggablePanel : MonoBehaviour, IPointerDownHandler, IBeginDragHand
     [Tooltip("Panel pixels (canvas units) kept on screen on every edge so it can never be lost.")]
     public float keepVisible = 48f;
 
+    // Raised once when a real panel drag ends (not for gestures that began on a widget). The HUD uses
+    // this to persist the layout the moment a panel is dropped. Optional - left null by other users.
+    public System.Action Moved;
+
     private RectTransform _area; // the panel's parent; pointer positions are tracked in its local space
     private Vector2 _grabOffset;
     private bool _dragging;
@@ -66,6 +70,13 @@ public class DraggablePanel : MonoBehaviour, IPointerDownHandler, IBeginDragHand
         target.anchoredPosition = Clamp(local + _grabOffset);
     }
 
+    public void OnEndDrag(PointerEventData eventData)
+    {
+        if (!_dragging) return; // gesture began on a widget -> not our drag, nothing moved
+        _dragging = false;
+        Moved?.Invoke();
+    }
+
     // True when the press started on an interactive widget inside the panel (which then owns the gesture).
     private bool StartedOnWidget(PointerEventData eventData)
     {

Assets/Scripts/PaintHud.Persistence.cs

@@ -7,10 +7,14 @@ public partial class PaintHud
 {
     // --- Persistence: panel positions + the widget-bound painter values ---
     //
-    // Saved on quit / play-stop and restored on the next start, so the HUD opens where you left it
-    // with the brush settings you last used. This is small structured state (a handful of positions
-    // and scalars), so it goes to a human-readable JSON file - unlike the painter's bulk pixel maps
-    // (MeshPainter.Persistence) which use a raw byte dump. The two saves are independent: this one
+    // Saved when state actually changes - a panel drop writes immediately (OnPanelMoved), brush-value
+    // edits are coalesced and flushed shortly after they settle (LateUpdate debounce) - and restored on
+    // the next start, so the HUD opens where you left it with the brush settings you last used. There is
+    // deliberately NO save on quit/teardown: standalone builds do not guarantee a clean shutdown
+    // (OnApplicationQuit is skipped on force-close / kill / crash), so a teardown-only save silently
+    // lost the whole session's layout in the build. This is small structured state (a handful of
+    // positions and scalars), so it goes to a human-readable JSON file - unlike the painter's bulk pixel
+    // maps (MeshPainter.Persistence) which use a raw byte dump. The two saves are independent: this one
     // owns the HUD layout + brush values, the painter owns the canvas pixels.
     //
     // The brush values are also mirrored into an in-memory snapshot (_carry) on this persistent HUD, so
@@ -20,8 +24,9 @@ public partial class PaintHud
     // additive lobby/map loads, which would split save and restore into different files.
 
     [Header("Persistence")]
-    [Tooltip("Save the panel positions and their values on quit / play-stop and restore them on the " +
-             "next start. Off = the HUD opens at its authored layout / painter defaults every session.")]
+    [Tooltip("Save the panel positions and their values when you move a panel or change a brush value, " +
+             "and restore them on the next start. Off = the HUD opens at its authored layout / painter " +
+             "defaults every session.")]
     public bool persistHud = true;
 
     [Tooltip("Save file identifier. Empty = derived from the HUD's own scene name. Set explicitly to keep " +
@@ -29,7 +34,15 @@ public partial class PaintHud
     public string hudSaveId = "";
 
     private const int HudFileVersion = 1;
-    private bool _hudSavedOnTeardown; // guards a double save across OnApplicationQuit + OnDestroy
+
+    // Auto-save bookkeeping. Panel drops save instantly; brush-value edits are debounced because sliders
+    // fire every frame while dragged - we push the due time forward and write once they settle. No
+    // teardown save, so persistence never depends on a clean quit.
+    private const float HudSaveDebounce = 0.5f;
+    private readonly HudValues _savedValues = new HudValues(); // mirror of what is currently on disk
+    private bool _hasSavedBaseline;
+    private bool _hudSavePending;
+    private float _hudSaveDueTime;
 
     // In-memory mirror of the brush values, kept on this persistent HUD so they survive the avatar
     // respawn a scene switch (lobby <-> map) triggers: the freshly spawned painter is seeded with these
@@ -87,15 +100,8 @@ public partial class PaintHud
         public float sprayDotSizeMin, sprayDotSizeMax, sprayDotAlphaMin, sprayDotAlphaMax;
     }
 
-    private void OnApplicationQuit() => SaveHudOnTeardown();
-
-    /// <summary>Saves once during teardown (OnApplicationQuit and OnDestroy both call this; only the first writes).</summary>
-    private void SaveHudOnTeardown()
-    {
-        if (_hudSavedOnTeardown) return;
-        _hudSavedOnTeardown = true;
-        SaveHudState();
-    }
+    /// <summary>A draggable panel was dropped -> persist the layout immediately.</summary>
+    private void OnPanelMoved() => SaveHudState();
 
     /// <summary>Writes the current panel positions and bound painter values to disk.</summary>
     public void SaveHudState()
@@ -127,6 +133,11 @@ public partial class PaintHud
 
             Directory.CreateDirectory(HudSaveDir);
             File.WriteAllText(HudSavePath(), JsonUtility.ToJson(state, true));
+
+            // Refresh the on-disk baseline so the debounce does not immediately re-save unchanged values.
+            if (painter) CopyValues(painter, _savedValues);
+            _hasSavedBaseline = painter;
+            _hudSavePending = false;
         }
         catch (Exception e)
         {
@@ -165,10 +176,57 @@ public partial class PaintHud
 
     private void LateUpdate()
     {
-        // Mirror the live painter every frame; the next BindPainter (after a respawn) seeds the fresh
-        // painter from this snapshot instead of its prefab defaults. Captured while the painter lives, so
-        // it is immune to the destroy-pending timing of the despawn.
-        if (painter) CaptureCarry();
+        if (painter)
+        {
+            // Mirror the live painter every frame; the next BindPainter (after a respawn) seeds the fresh
+            // painter from this snapshot instead of its prefab defaults. Captured while the painter lives,
+            // so it is immune to the destroy-pending timing of the despawn.
+            CaptureCarry();
+
+            // A brush-value edit differs from what is on disk -> schedule a coalesced save. Sliders fire
+            // every frame while dragged, so we keep pushing the due time forward and write once they settle.
+            if (persistHud && _hasSavedBaseline && !ValuesEqual(painter, _savedValues))
+            {
+                _hudSavePending = true;
+                _hudSaveDueTime = Time.unscaledTime + HudSaveDebounce;
+            }
+        }
+
+        if (_hudSavePending && Time.unscaledTime >= _hudSaveDueTime)
+            SaveHudState();
+    }
+
+    /// <summary>Marks the current brush values as the on-disk baseline so auto-save only fires on real edits.</summary>
+    private void SnapshotSavedBaseline()
+    {
+        if (painter) CopyValues(painter, _savedValues);
+        _hasSavedBaseline = painter;
+        _hudSavePending = false;
+    }
+
+    /// <summary>True when every brush value on the painter still matches the snapshot last written to disk.</summary>
+    private static bool ValuesEqual(MeshPainter p, HudValues v)
+    {
+        return p.paintAlbedo == v.paintAlbedo
+            && p.paintMetallicSmoothness == v.paintMetallicSmoothness
+            && p.paintOcclusion == v.paintOcclusion
+            && p.paintEmission == v.paintEmission
+            && p.spray == v.spray
+            && p.brushColor == v.brushColor
+            && p.metallic == v.metallic
+            && p.smoothness == v.smoothness
+            && p.metallicSmoothnessOpacity == v.metallicSmoothnessOpacity
+            && p.occlusion == v.occlusion
+            && p.occlusionOpacity == v.occlusionOpacity
+            && p.emissionColor == v.emissionColor
+            && p.emissionIntensity == v.emissionIntensity
+            && p.brushSize == v.brushSize
+            && p.activeBrushIndex == v.activeBrushIndex
+            && p.sprayDots == v.sprayDots
+            && p.sprayDotSizeMin == v.sprayDotSizeMin
+            && p.sprayDotSizeMax == v.sprayDotSizeMax
+            && p.sprayDotAlphaMin == v.sprayDotAlphaMin
+            && p.sprayDotAlphaMax == v.sprayDotAlphaMax;
     }
 
     /// <summary>Snapshots the bound painter's brush values into the in-memory mirror.</summary>

Assets/Scripts/PaintHud.cs

@@ -162,6 +162,7 @@ public partial class PaintHud : MonoBehaviour
         if (_hasCarry) ApplyCarry();
         else RestoreHudState();
         CaptureCarry(); // seed/refresh the mirror from the now-current painter
+        SnapshotSavedBaseline(); // values now match disk -> auto-save only fires on a real edit
 
         UpdateButtonStates(); // enable Save now that a painter is bound
     }
@@ -169,7 +170,6 @@ public partial class PaintHud : MonoBehaviour
     private void OnDestroy()
     {
         if (painter) painter.Picked -= SyncFromPainter;
-        SaveHudOnTeardown();
     }
 
     private void OnModeChanged(GameMode mode)
@@ -409,13 +409,14 @@ public partial class PaintHud : MonoBehaviour
     /// from any of its non-interactive surface (header, padding, labels, background); the component
     /// ignores presses that start on a widget (toggle, slider, colour picker).
     /// </summary>
-    private static void MakeDraggable(Transform root, string panelName)
+    private void MakeDraggable(Transform root, string panelName)
     {
         var panel = FindDeep(root, panelName) as RectTransform;
         if (!panel) return;
         var drag = panel.GetComponent<DraggablePanel>();
         if (!drag) drag = panel.gameObject.AddComponent<DraggablePanel>();
         drag.target = panel;
+        drag.Moved = OnPanelMoved; // persist the layout the moment a panel is dropped
     }
 
     private void EnsureFonts(Transform panel)