diff --git a/widget/gtk/WidgetStyleCache.cpp b/widget/gtk/WidgetStyleCache.cpp
index c5bfbd55b900..41c5111e319c 100644
--- a/widget/gtk/WidgetStyleCache.cpp
+++ b/widget/gtk/WidgetStyleCache.cpp
@@ -555,6 +555,14 @@ CreateHeaderBar(WidgetNodeType aWidgetType)
   // Emulate what create_titlebar() at gtkwindow.c does.
   GtkStyleContext* style = gtk_widget_get_style_context(headerbar);
   gtk_style_context_add_class(style, "titlebar");
+
+  // TODO: Define default-decoration titlebar style as workaround
+  // to ensure the titlebar buttons does not overflow outside.
+  // Recently the titlebar size is calculated as
+  // tab size + titlebar border/padding (default-decoration has 6px padding
+  // at default Adwaita theme).
+  // We need to fix titlebar size calculation to also include
+  // titlebar button sizes. (Bug 1419442)
   gtk_style_context_add_class(style, "default-decoration");
 
   return headerbar;
diff --git a/widget/gtk/nsNativeThemeGTK.cpp b/widget/gtk/nsNativeThemeGTK.cpp
index cc40f77b69b0..c82f25650f8d 100644
--- a/widget/gtk/nsNativeThemeGTK.cpp
+++ b/widget/gtk/nsNativeThemeGTK.cpp
@@ -1575,6 +1575,16 @@ nsNativeThemeGTK::GetMinimumWidgetSize(nsPresContext* aPresContext,
       *aIsOverridable = false;
     }
     break;
+  case NS_THEME_WINDOW_BUTTON_CLOSE:
+  case NS_THEME_WINDOW_BUTTON_MINIMIZE:
+  case NS_THEME_WINDOW_BUTTON_MAXIMIZE:
+  case NS_THEME_WINDOW_BUTTON_RESTORE:
+    {
+      const ToolbarButtonGTKMetrics* metrics = GetToolbarButtonMetrics();
+      aResult->width = metrics->minSizeWithBorderMargin.width;
+      aResult->height = metrics->minSizeWithBorderMargin.height;
+      break;
+    }
   case NS_THEME_CHECKBOX_CONTAINER:
   case NS_THEME_RADIO_CONTAINER:
   case NS_THEME_CHECKBOX_LABEL:
@@ -1583,10 +1593,6 @@ nsNativeThemeGTK::GetMinimumWidgetSize(nsPresContext* aPresContext,
   case NS_THEME_MENULIST:
   case NS_THEME_TOOLBARBUTTON:
   case NS_THEME_TREEHEADERCELL:
-  case NS_THEME_WINDOW_BUTTON_CLOSE:
-  case NS_THEME_WINDOW_BUTTON_MINIMIZE:
-  case NS_THEME_WINDOW_BUTTON_MAXIMIZE:
-  case NS_THEME_WINDOW_BUTTON_RESTORE:
     {
       if (aWidgetType == NS_THEME_MENULIST) {
         // Include the arrow size.