FocusWindow(NULL) correctly steal active id from previous window. (#1738) amend b0a9bbf6

imgui.cpp

@@ -6161,20 +6161,19 @@ void ImGui::FocusWindow(ImGuiWindow* window)
     // Close popups if any
     ClosePopupsOverWindow(window, false);
 
-    // Passing NULL allow to disable keyboard focus
-    if (!window)
-        return;
-
     // Move the root window to the top of the pile
-    IM_ASSERT(window->RootWindow != NULL);
-    ImGuiWindow* focus_front_window = window->RootWindow; // NB: In docking branch this is window->RootWindowDockStop
-    ImGuiWindow* display_front_window = window->RootWindow;
+    IM_ASSERT(window == NULL || window->RootWindow != NULL);
+    ImGuiWindow* focus_front_window = window ? window->RootWindow : NULL; // NB: In docking branch this is window->RootWindowDockStop
+    ImGuiWindow* display_front_window = window ? window->RootWindow : NULL;
 
     // Steal focus on active widgets
-    if (focus_front_window->Flags & ImGuiWindowFlags_Popup) // FIXME: This statement may be unnecessary? Need further testing before removing it..
     if (g.ActiveId != 0 && g.ActiveIdWindow && g.ActiveIdWindow->RootWindow != focus_front_window)
         ClearActiveID();
 
+    // Passing NULL allow to disable keyboard focus
+    if (!window)
+        return;
+
     // Bring to front
     BringWindowToFocusFront(focus_front_window);
     if (((window->Flags | display_front_window->Flags) & ImGuiWindowFlags_NoBringToFrontOnFocus) == 0)