Made it illegal/assert when io.DisplayTime == 0.0f (with an exception for the first frame).

Causing too many subtle side-effect, e.g. IsNavInputPressed() would return true multiple times in a row.

docs/CHANGELOG.txt

@@ -36,6 +36,8 @@ HOW TO UPDATE?
 Breaking Changes:
 
 - Removed io.DisplayVisibleMin/DisplayVisibleMax (which were marked obsolete and removed from viewport/docking branch already).
+- Made it illegal/assert when io.DisplayTime == 0.0f (with an exception for the first frame).
+  If for some reason your time step calculation gives you a zero value, replace it with a dummy small value!
 
 Other Changes:
 - Added .editorconfig file for text editors to standardize using spaces. (#2038) [@kudaba]

imgui.cpp

@@ -364,6 +364,7 @@ CODE
  When you are not sure about a old symbol or function name, try using the Search/Find function of your IDE to look for comments or references in all imgui files.
  You can read releases logs https://github.com/ocornut/imgui/releases for more details.
 
+ - 2019/02/14 (1.68) - made it illegal/assert when io.DisplayTime == 0.0f (with an exception for the first frame). If for some reason your time step calculation gives you a zero value, replace it with a dummy small value!
  - 2019/02/01 (1.68) - removed io.DisplayVisibleMin/DisplayVisibleMax (which were marked obsolete and removed from viewport/docking branch already).
  - 2019/01/06 (1.67) - renamed io.InputCharacters[], marked internal as was always intended. Please don't access directly, and use AddInputCharacter() instead!
  - 2019/01/06 (1.67) - renamed ImFontAtlas::GlyphRangesBuilder to ImFontGlyphRangesBuilder. Keep redirection typedef (will obsolete).
@@ -3364,12 +3365,12 @@ void ImGui::NewFrame()
     // Check user data
     // (We pass an error message in the assert expression to make it visible to programmers who are not using a debugger, as most assert handlers display their argument)
     IM_ASSERT(g.Initialized);
-    IM_ASSERT(g.IO.DeltaTime >= 0.0f                                    && "Need a positive DeltaTime (zero is tolerated but will cause some timing issues)");
-    IM_ASSERT(g.IO.DisplaySize.x >= 0.0f && g.IO.DisplaySize.y >= 0.0f  && "Invalid DisplaySize value");
+    IM_ASSERT((g.IO.DeltaTime > 0.0f || g.FrameCount == 0)              && "Need a positive DeltaTime!");
+    IM_ASSERT(g.IO.DisplaySize.x >= 0.0f && g.IO.DisplaySize.y >= 0.0f  && "Invalid DisplaySize value!");
     IM_ASSERT(g.IO.Fonts->Fonts.Size > 0                                && "Font Atlas not built. Did you call io.Fonts->GetTexDataAsRGBA32() / GetTexDataAsAlpha8() ?");
     IM_ASSERT(g.IO.Fonts->Fonts[0]->IsLoaded()                          && "Font Atlas not built. Did you call io.Fonts->GetTexDataAsRGBA32() / GetTexDataAsAlpha8() ?");
-    IM_ASSERT(g.Style.CurveTessellationTol > 0.0f                       && "Invalid style setting");
-    IM_ASSERT(g.Style.Alpha >= 0.0f && g.Style.Alpha <= 1.0f            && "Invalid style setting. Alpha cannot be negative (allows us to avoid a few clamps in color computations)");
+    IM_ASSERT(g.Style.CurveTessellationTol > 0.0f                       && "Invalid style setting!");
+    IM_ASSERT(g.Style.Alpha >= 0.0f && g.Style.Alpha <= 1.0f            && "Invalid style setting. Alpha cannot be negative (allows us to avoid a few clamps in color computations)!");
     IM_ASSERT((g.FrameCount == 0 || g.FrameCountEnded == g.FrameCount)  && "Forgot to call Render() or EndFrame() at the end of the previous frame?");
     for (int n = 0; n < ImGuiKey_COUNT; n++)
         IM_ASSERT(g.IO.KeyMap[n] >= -1 && g.IO.KeyMap[n] < IM_ARRAYSIZE(g.IO.KeysDown) && "io.KeyMap[] contains an out of bound value (need to be 0..512, or -1 for unmapped key)");