This has only been tested on trunk so far. Not sure when it started.

1) Ensure repaint dots are enabled in the Preferences window (under Developer Mode)

2) Notice the that the Project Browser busy repaints even when not needed.

3) You can enable the Diagnostics Switch: Editor->Repain Requested to see the callstack of where the repaint is requested every frame. See below.

{{// Repaint again if we are in preview icon mode and previews are being loaded}}
{{if (!ListMode && AssetPreview.IsLoadingAssetPreviews(...)}}
{{     m_Owner.Repaint();}}

Callstack: Repaint requested for ProjectBrowser.

Check the call stack to find the root cause. Ensure 'Stack Trace Logging -> Log -> Full' is enabled in the Console window menu.

Repaint requested for ProjectBrowser.

Check the call stack to find the root cause. Ensure 'Stack Trace Logging -> Log -> Full' is enabled in the Console window menu.
0x00007ff7d4326688 (Unity) [StackWalker.cpp:1005] StackWalker::GetCurrentCallstack 
0x00007ff7d43388ac (Unity) [StackWalker.cpp:983] StackWalker::ShowCallstack 
0x00007ff7d8911ffd (Unity) [Stacktrace.cpp:166] GetStacktrace 
0x00007ff7dad2f090 (Unity) [LogAssert.cpp:1159] DebugStringToFile 
0x00007ff7d0774bc8 (Unity) [LogAssert.h:149] DebugStringToFile<core::basic_string<char,core::StringStorageDefault<char> > > 
0x00007ff7d6eb6395 (Unity) [EditorWindowsShared.cpp:211] GUIView::RequestRepaint 
0x00007ff7d6df441f (Unity) [GUIView.bindings.cpp:187] MonoGUIView::Repaint 
0x00007ff7d0c99853 (Unity) [EditorBindings.gen.cpp:51764] GUIView_CUSTOM_Repaint 
0x000001af91021b69 (Mono JIT Code) (wrapper managed-to-native) UnityEditor.GUIView:Repaint (UnityEditor.GUIView)
0x000001af911fa84b (Mono JIT Code) [EditorWindow.cs:1027] UnityEditor.EditorWindow:Repaint () 
0x000001af8d657ee0 (Mono JIT Code) [ObjectListArea.cs:487] UnityEditor.ObjectListArea:Repaint () 
0x000001af8d6782e3 (Mono JIT Code) [ObjectListLocalGroup.cs:265] UnityEditor.ObjectListArea/LocalGroup:DrawInternal (int,int,single) 
0x000001af8d676762 (Mono JIT Code) [ObjectListGroup.cs:152] UnityEditor.ObjectListArea/Group:Draw (single,UnityEngine.Vector2,int&) 
0x000001af8d67511b (Mono JIT Code) [ObjectListArea.cs:1090] UnityEditor.ObjectListArea:HandleListArea () 
0x000001af8d671d8b (Mono JIT Code) [ObjectListArea.cs:396] UnityEditor.ObjectListArea:OnGUI (UnityEngine.Rect,int) 
0x000001af8d5faeab (Mono JIT Code) [ProjectBrowser.cs:2061] UnityEditor.ProjectBrowser:OnGUI ()