Tweaked asserts

examples/imgui_impl_glfw.cpp

@@ -267,9 +267,9 @@ static void ImGui_ImplGlfw_UpdateMouseCursor()
 void ImGui_ImplGlfw_NewFrame()
 {
     ImGuiIO& io = ImGui::GetIO();
-    IM_ASSERT(io.Fonts->IsBuilt());     // Font atlas needs to be built, call renderer _NewFrame() function e.g. ImGui_ImplOpenGL3_NewFrame() 
+    IM_ASSERT(io.Fonts->IsBuilt() && "Font atlas not built! It is generally built by the renderer back-end. Missing call to renderer _NewFrame() function? e.g. ImGui_ImplOpenGL3_NewFrame().");
 
-    // Setup display size
+    // Setup display size (every frame to accommodate for window resizing)
     int w, h;
     int display_w, display_h;
     glfwGetWindowSize(g_Window, &w, &h);

examples/imgui_impl_sdl.cpp

@@ -271,7 +271,7 @@ static void ImGui_ImplSDL2_UpdateMouseCursor()
 void ImGui_ImplSDL2_NewFrame(SDL_Window* window)
 {
     ImGuiIO& io = ImGui::GetIO();
-    IM_ASSERT(io.Fonts->IsBuilt());     // Font atlas needs to be built, call renderer _NewFrame() function e.g. ImGui_ImplOpenGL3_NewFrame() 
+    IM_ASSERT(io.Fonts->IsBuilt() && "Font atlas not built! It is generally built by the renderer back-end. Missing call to renderer _NewFrame() function? e.g. ImGui_ImplOpenGL3_NewFrame().");
 
     // Setup display size (every frame to accommodate for window resizing)
     int w, h;

examples/imgui_impl_win32.cpp

@@ -142,6 +142,7 @@ static void ImGui_ImplWin32_UpdateMousePos()
 void    ImGui_ImplWin32_NewFrame()
 {
     ImGuiIO& io = ImGui::GetIO();
+    IM_ASSERT(io.Fonts->IsBuilt() && "Font atlas not built! It is generally built by the renderer back-end. Missing call to renderer _NewFrame() function? e.g. ImGui_ImplOpenGL3_NewFrame().");
 
     // Setup display size (every frame to accommodate for window resizing)
     RECT rect;

imgui.cpp

@@ -3553,7 +3553,7 @@ void ImGui::EndFrame()
     IM_ASSERT(g.Initialized);
     if (g.FrameCountEnded == g.FrameCount)          // Don't process EndFrame() multiple times.
         return;
-    IM_ASSERT(g.FrameScopeActive && "Forgot to call ImGui::NewFrame()");
+    IM_ASSERT(g.FrameScopeActive && "Forgot to call ImGui::NewFrame()?");
 
     // Notify OS when our Input Method Editor cursor has moved (e.g. CJK inputs using Microsoft IME)
     if (g.IO.ImeSetInputScreenPosFn && ImLengthSqr(g.PlatformImeLastPos - g.PlatformImePos) > 0.0001f)
@@ -3562,8 +3562,17 @@ void ImGui::EndFrame()
         g.PlatformImeLastPos = g.PlatformImePos;
     }
 
-    // Hide implicit "Debug" window if it hasn't been used
-    IM_ASSERT(g.CurrentWindowStack.Size == 1);    // Mismatched Begin()/End() calls, did you forget to call end on g.CurrentWindow->Name?
+    // Report when there is a mismatch of Begin/BeginChild vs End/EndChild calls. Important: Remember that the Begin/BeginChild API requires you
+    // to always call End/EndChild even if Begin/BeginChild returns false! (this is unfortunately inconsistent with most other Begin* API).
+    if (g.CurrentWindowStack.Size != 1)
+    {
+        if (g.CurrentWindowStack.Size > 1)
+            IM_ASSERT(g.CurrentWindowStack.Size == 1 && "Mismatched Begin/BeginChild vs End/EndChild calls: did you forget to call End/EndChild?");
+        else
+            IM_ASSERT(g.CurrentWindowStack.Size == 1 && "Mismatched Begin/BeginChild vs End/EndChild calls: did you call End/EndChild too much?");
+    }
+
+    // Hide implicit/fallback "Debug" window if it hasn't been used
     if (g.CurrentWindow && !g.CurrentWindow->WriteAccessed)
         g.CurrentWindow->Active = false;
     End();
@@ -3699,7 +3708,7 @@ void ImGui::Render()
     g.IO.MetricsRenderVertices = g.DrawData.TotalVtxCount;
     g.IO.MetricsRenderIndices = g.DrawData.TotalIdxCount;
 
-    // Render. If user hasn't set a callback then they may retrieve the draw data via GetDrawData()
+    // (Legacy) Call the Render callback function. The current prefer way is to let the user retrieve GetDrawData() and call the render function themselves.
 #ifndef IMGUI_DISABLE_OBSOLETE_FUNCTIONS
     if (g.DrawData.CmdListsCount > 0 && g.IO.RenderDrawListsFn != NULL)
         g.IO.RenderDrawListsFn(&g.DrawData);