diff --git a/src/app/ui/status_bar.cpp b/src/app/ui/status_bar.cpp
index e3d88846e..ce9007160 100644
--- a/src/app/ui/status_bar.cpp
+++ b/src/app/ui/status_bar.cpp
@@ -801,6 +801,13 @@ void StatusBar::onRemoveDocument(doc::Document* doc)
 
 void StatusBar::onPixelFormatChanged(DocumentEvent& ev)
 {
+  // If this is called from the non-UI thread it means that the pixel
+  // format change was made in the background,
+  // i.e. ChangePixelFormatCommand uses a background thread to change
+  // the sprite format.
+  if (!ui::is_ui_thread())
+    return;
+
   onActiveSiteChange(UIContext::instance()->activeSite());
 }
 
diff --git a/src/ui/system.cpp b/src/ui/system.cpp
index 0282f9c9a..981a7bed5 100644
--- a/src/ui/system.cpp
+++ b/src/ui/system.cpp
@@ -10,6 +10,7 @@
 
 #include "ui/system.h"
 
+#include "base/thread.h"
 #include "gfx/point.h"
 #include "she/display.h"
 #include "she/surface.h"
@@ -26,6 +27,10 @@
 
 namespace ui {
 
+// This is used to check if calls to UI layer are made from the non-UI
+// thread. (Which might be catastrofic.)
+base::thread::native_handle_type main_gui_thread;
+
 // Current mouse cursor type.
 
 static CursorType mouse_cursor_type = kOutsideDisplay;
@@ -168,6 +173,7 @@ static void update_mouse_cursor()
 
 UISystem::UISystem()
 {
+  main_gui_thread = base::this_thread::native_handle();
   mouse_cursor_type = kOutsideDisplay;
   support_native_custom_cursor =
     ((she::instance() &&
@@ -306,4 +312,16 @@ void set_mouse_position(const gfx::Point& newPos)
   _internal_set_mouse_position(newPos);
 }
 
+bool is_ui_thread()
+{
+  return (main_gui_thread == base::this_thread::native_handle());
+}
+
+#ifdef _DEBUG
+void assert_ui_thread()
+{
+  ASSERT(is_ui_thread());
+}
+#endif
+
 } // namespace ui
diff --git a/src/ui/system.h b/src/ui/system.h
index cb5c6bd2d..06e76d3af 100644
--- a/src/ui/system.h
+++ b/src/ui/system.h
@@ -52,6 +52,13 @@ namespace ui {
   const gfx::Point& get_mouse_position();
   void set_mouse_position(const gfx::Point& newPos);
 
+  bool is_ui_thread();
+#ifdef _DEBUG
+  void assert_ui_thread();
+#else
+  static inline void assert_ui_thread() { }
+#endif
+
 } // namespace ui
 
 #endif
diff --git a/src/ui/widget.cpp b/src/ui/widget.cpp
index 63501d1b9..4e9cc74f8 100644
--- a/src/ui/widget.cpp
+++ b/src/ui/widget.cpp
@@ -156,6 +156,8 @@ void Widget::setTextf(const char *format, ...)
 
 void Widget::setTextQuiet(const std::string& text)
 {
+  assert_ui_thread();
+
   m_text = text;
   enableFlags(HAS_TEXT);
 }
@@ -169,6 +171,8 @@ she::Font* Widget::font() const
 
 void Widget::setBgColor(gfx::Color color)
 {
+  assert_ui_thread();
+
   m_bgColor = color;
   onSetBgColor();
 
@@ -179,6 +183,8 @@ void Widget::setBgColor(gfx::Color color)
 
 void Widget::setTheme(Theme* theme)
 {
+  assert_ui_thread();
+
   m_theme = theme;
   m_font = nullptr;
 
@@ -188,6 +194,8 @@ void Widget::setTheme(Theme* theme)
 
 void Widget::setStyle(Style* style)
 {
+  assert_ui_thread();
+
   m_style = style;
   m_border = m_theme->calcBorder(this, style);
   m_bgColor = m_theme->calcBgColor(this, style);
@@ -201,6 +209,8 @@ void Widget::setStyle(Style* style)
 
 void Widget::setVisible(bool state)
 {
+  assert_ui_thread();
+
   if (state) {
     if (hasFlags(HIDDEN)) {
       disableFlags(HIDDEN);
@@ -221,6 +231,8 @@ void Widget::setVisible(bool state)
 
 void Widget::setEnabled(bool state)
 {
+  assert_ui_thread();
+
   if (state) {
     if (hasFlags(DISABLED)) {
       disableFlags(DISABLED);
@@ -243,6 +255,8 @@ void Widget::setEnabled(bool state)
 
 void Widget::setSelected(bool state)
 {
+  assert_ui_thread();
+
   if (state) {
     if (!hasFlags(SELECTED)) {
       enableFlags(SELECTED);
@@ -263,6 +277,8 @@ void Widget::setSelected(bool state)
 
 void Widget::setExpansive(bool state)
 {
+  assert_ui_thread();
+
   if (state)
     enableFlags(EXPANSIVE);
   else
@@ -271,6 +287,8 @@ void Widget::setExpansive(bool state)
 
 void Widget::setDecorative(bool state)
 {
+  assert_ui_thread();
+
   if (state)
     enableFlags(DECORATIVE);
   else
@@ -279,6 +297,8 @@ void Widget::setDecorative(bool state)
 
 void Widget::setFocusStop(bool state)
 {
+  assert_ui_thread();
+
   if (state)
     enableFlags(FOCUS_STOP);
   else
@@ -287,6 +307,8 @@ void Widget::setFocusStop(bool state)
 
 void Widget::setFocusMagnet(bool state)
 {
+  assert_ui_thread();
+
   if (state)
     enableFlags(FOCUS_MAGNET);
   else
@@ -295,6 +317,8 @@ void Widget::setFocusMagnet(bool state)
 
 bool Widget::isVisible() const
 {
+  assert_ui_thread();
+
   const Widget* widget = this;
   const Widget* lastWidget = nullptr;
 
@@ -312,6 +336,8 @@ bool Widget::isVisible() const
 
 bool Widget::isEnabled() const
 {
+  assert_ui_thread();
+
   const Widget* widget = this;
 
   do {
@@ -326,26 +352,31 @@ bool Widget::isEnabled() const
 
 bool Widget::isSelected() const
 {
+  assert_ui_thread();
   return hasFlags(SELECTED);
 }
 
 bool Widget::isExpansive() const
 {
+  assert_ui_thread();
   return hasFlags(EXPANSIVE);
 }
 
 bool Widget::isDecorative() const
 {
+  assert_ui_thread();
   return hasFlags(DECORATIVE);
 }
 
 bool Widget::isFocusStop() const
 {
+  assert_ui_thread();
   return hasFlags(FOCUS_STOP);
 }
 
 bool Widget::isFocusMagnet() const
 {
+  assert_ui_thread();
   return hasFlags(FOCUS_MAGNET);
 }
 
@@ -355,6 +386,8 @@ bool Widget::isFocusMagnet() const
 
 Window* Widget::window() const
 {
+  assert_ui_thread();
+
   const Widget* widget = this;
 
   while (widget) {
@@ -369,6 +402,8 @@ Window* Widget::window() const
 
 Manager* Widget::manager() const
 {
+  assert_ui_thread();
+
   const Widget* widget = this;
 
   while (widget) {
@@ -383,6 +418,8 @@ Manager* Widget::manager() const
 
 void Widget::getParents(bool ascendant, WidgetsList& parents)
 {
+  assert_ui_thread();
+
   for (Widget* widget=this; widget; widget=widget->m_parent) {
     // append parents in tail
     if (ascendant)
@@ -395,6 +432,8 @@ void Widget::getParents(bool ascendant, WidgetsList& parents)
 
 Widget* Widget::nextSibling()
 {
+  assert_ui_thread();
+
   if (!m_parent)
     return NULL;
 
@@ -413,6 +452,8 @@ Widget* Widget::nextSibling()
 
 Widget* Widget::previousSibling()
 {
+  assert_ui_thread();
+
   if (!m_parent)
     return NULL;
 
@@ -429,6 +470,8 @@ Widget* Widget::previousSibling()
 Widget* Widget::pick(const gfx::Point& pt,
                      const bool checkParentsVisibility) const
 {
+  assert_ui_thread();
+
   const Widget* inside, *picked = nullptr;
 
   // isVisible() checks visibility of widget's parent.
@@ -452,6 +495,7 @@ Widget* Widget::pick(const gfx::Point& pt,
 bool Widget::hasChild(Widget* child)
 {
   ASSERT_VALID_WIDGET(child);
+  assert_ui_thread();
 
   return std::find(m_children.begin(), m_children.end(), child) != m_children.end();
 }