Made assertion more clear when trying to call Begin() outside of the NewFrame()..EndFrame() scope. (#1987)

CHANGELOG.txt

@@ -56,6 +56,7 @@ Other Changes:
  - IsItemHovered(): Added ImGuiHoveredFlags_AllowWhenDisabled flag to query hovered status on disabled items. (#1940, #211)
  - Misc: Added ImGuiMouseCursor_Hand cursor enum + corresponding software cursor. (#1913, 1914) [@aiekick, @ocornut] 
  - Misc: Tweaked software mouse cursor offset to match the offset of the corresponding Windows 10 cursors.
+ - Made assertion more clear when trying to call Begin() outside of the NewFrame()..EndFrame() scope. (#1987)
  - Fixed horizontal mouse wheel not forwarding the request to the parent window if ImGuiWindowFlags_NoScrollWithMouse is set. (#1463, #1380, #1502)
  - Fixed a include build issue for Cygwin in non-POSIX (Win32) mode. (#1917, #1319, #276)
  - OS/Windows: Fixed missing ImmReleaseContext() call in the default Win32 IME handler. (#1932) [@vby]

imgui.cpp

@@ -3883,6 +3883,7 @@ void ImGui::NewFrame()
     }
 
     g.Time += g.IO.DeltaTime;
+    g.FrameScopeActive = true;
     g.FrameCount += 1;
     g.TooltipOverrideCount = 0;
     g.WindowsActiveCount = 0;
@@ -4421,9 +4422,11 @@ void ImGui::PopClipRect()
 void ImGui::EndFrame()
 {
     ImGuiContext& g = *GImGui;
-    IM_ASSERT(g.Initialized);                       // Forgot to call ImGui::NewFrame()
+    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()");
+    g.FrameScopeActive = false;
 
     // 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)
@@ -4508,7 +4511,7 @@ void ImGui::EndFrame()
 void ImGui::Render()
 {
     ImGuiContext& g = *GImGui;
-    IM_ASSERT(g.Initialized);   // Forgot to call ImGui::NewFrame()
+    IM_ASSERT(g.Initialized);
 
     if (g.FrameCountEnded != g.FrameCount)
         ImGui::EndFrame();
@@ -6212,7 +6215,7 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
     ImGuiContext& g = *GImGui;
     const ImGuiStyle& style = g.Style;
     IM_ASSERT(name != NULL);                        // Window name required
-    IM_ASSERT(g.Initialized);                       // Forgot to call ImGui::NewFrame()
+    IM_ASSERT(g.FrameScopeActive);                  // Forgot to call ImGui::NewFrame()
     IM_ASSERT(g.FrameCountEnded != g.FrameCount);   // Called ImGui::Render() or ImGui::EndFrame() and haven't called ImGui::NewFrame() again yet
 
     // Find or create

imgui_internal.h

@@ -608,6 +608,7 @@ struct ImGuiNextWindowData
 struct ImGuiContext
 {
     bool                    Initialized;
+    bool                    FrameScopeActive;                   // Set by NewFrame(), cleared by EndFrame()/Render()
     bool                    FontAtlasOwnedByContext;            // Io.Fonts-> is owned by the ImGuiContext and will be destructed along with it.
     ImGuiIO                 IO;
     ImGuiStyle              Style;
@@ -761,6 +762,7 @@ struct ImGuiContext
     ImGuiContext(ImFontAtlas* shared_font_atlas) : OverlayDrawList(NULL)
     {
         Initialized = false;
+        FrameScopeActive = false;
         Font = NULL;
         FontSize = FontBaseSize = 0.0f;
         FontAtlasOwnedByContext = shared_font_atlas ? false : true;