Merge remote-tracking branch 'origin/master' into 1.9

# Conflicts:
#	Source/Editor/Modules/ContentDatabaseModule.cs
#	Source/Editor/Surface/SurfaceUtils.cs
#	Source/Editor/Windows/Assets/MaterialInstanceWindow.cs
#	Source/Engine/Foliage/Foliage.cpp
#	Source/Engine/Graphics/Models/MeshBase.h
#	Source/Engine/Tools/MaterialGenerator/MaterialGenerator.Textures.cpp

Source/Editor/GUI/ContextMenu/ContextMenu.cs

@@ -337,14 +337,12 @@ namespace FlaxEditor.GUI.ContextMenu
         /// <summary>
         /// Adds the separator.
         /// </summary>
-        /// <returns>Created context menu item control.</returns>
-        public ContextMenuSeparator AddSeparator()
+        public void AddSeparator()
         {
             var item = new ContextMenuSeparator(this)
             {
                 Parent = _panel
             };
-            return item;
         }
 
         /// <summary>

Source/Editor/GUI/EnumComboBox.cs

@@ -7,6 +7,7 @@ using System.Reflection;
 using System.Runtime.InteropServices;
 using FlaxEditor.CustomEditors;
 using FlaxEditor.CustomEditors.Elements;
+using FlaxEditor.GUI.ContextMenu;
 using FlaxEditor.Scripting;
 using FlaxEngine;
 
@@ -277,6 +278,14 @@ namespace FlaxEditor.GUI
             }
         }
 
+        /// <inheritdoc />
+        protected override void OnLayoutMenuButton(ContextMenuButton button, int index, bool construct = false)
+        {
+            base.OnLayoutMenuButton(button, index, construct);
+            if (IsFlags)
+                button.CloseMenuOnClick = false;
+        }
+
         /// <inheritdoc />
         protected override void OnItemClicked(int index)
         {

Source/Editor/GUI/Input/ValueBox.cs

@@ -195,7 +195,7 @@ namespace FlaxEditor.GUI.Input
                 var style = Style.Current;
 
                 // Draw sliding UI
-                Render2D.DrawSprite(style.Scalar, SlideRect, style.Foreground);
+                Render2D.DrawSprite(style.Scalar, SlideRect, EnabledInHierarchy ? style.Foreground : style.ForegroundDisabled);
 
                 // Check if is sliding
                 if (_isSliding)

Source/Editor/GUI/Timeline/AnimationTimeline.cs

@@ -130,9 +130,9 @@ namespace FlaxEditor.GUI.Timeline
         public override void OnPlay()
         {
             var time = CurrentTime;
-            _preview.Play();
             if (_preview != null)
             {
+                _preview.Play();
                 Editor.Internal_SetAnimationTime(Object.GetUnmanagedPtr(_preview.PreviewActor), time);
             }
 

Source/Editor/GUI/Timeline/Undo/EditTrackAction.cs

@@ -34,6 +34,8 @@ namespace FlaxEditor.GUI.Timeline.Undo
 
         private void Set(byte[] data)
         {
+            if (_timeline == null)
+                return;
             var track = _timeline.FindTrack(_name);
             using (var memory = new MemoryStream(data))
             using (var stream = new BinaryReader(memory))
@@ -42,11 +44,8 @@ namespace FlaxEditor.GUI.Timeline.Undo
                 track.Flags = (TrackFlags)stream.ReadByte();
                 track.Archetype.Load(Timeline.FormatVersion, track, stream);
             }
-            if (_timeline != null)
-            {
-                _timeline.ArrangeTracks();
-                _timeline.MarkAsEdited();
-            }
+            _timeline.ArrangeTracks();
+            _timeline.MarkAsEdited();
             track.OnUndo();
         }