Misc: Tolerate zero delta-time under Emscripten. (#6114, #3644)

docs/CHANGELOG.txt

@@ -51,6 +51,9 @@ All changes:
 - PlotHistogram, PlotLines: Passing negative sizes honor alignment like other widgets.
 - ImDrawList: Added missing early-out in AddPolyline() and AddConvexPolyFilled() when
   color alpha is zero.
+- Misc: Tolerate zero delta-time under Emscripten as backends are imprecise in their
+  values for io.DeltaTime, and browser features such as "privacy.resistFingerprinting=true"
+  can exacerbate that. (#6114, #3644)
 - Examples: Win32: Fixed examples using RegisterClassW() since 1.89 to also call
   DefWindowProcW() instead of DefWindowProc() so that title text are correctly converted
   when application is compiled without /DUNICODE. (#5725, #5961, #5975) [@markreidvfx]

imgui.cpp

@@ -8852,6 +8852,13 @@ static void ImGui::ErrorCheckNewFrameSanityChecks()
     // #define IM_ASSERT(EXPR)   do { if (SomeCode(EXPR)) SomeMoreCode(); } while (0)   // Correct!
     if (true) IM_ASSERT(1); else IM_ASSERT(0);
 
+    // Emscripten backends are often imprecise in their submission of DeltaTime. (#6114, #3644)
+    // Ideally the Emscripten app/backend should aim to fix or smooth this value and avoid feeding zero, but we tolerate it.
+#ifdef __EMSCRIPTEN__
+    if (g.IO.DeltaTime <= 0.0f && g.FrameCount > 0)
+        g.IO.DeltaTime = 0.00001f;
+#endif
+
     // 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);