Fonts: clarify assert. (#8680)

docs/CHANGELOG.txt

@@ -41,11 +41,14 @@ HOW TO UPDATE?
 
 THIS VERSION CONTAINS THE LARGEST AMOUNT OF BREAKING CHANGES SINCE 2015!
 I TRIED REALLY HARD TO KEEP THEM TO A MINIMUM, REDUCE THE AMOUNT OF INTERFERENCES,
-BUT INEVITABLY SOME USERS WILL BE AFFECTED.
+BUT INEVITABLY SOME USERS OR THIRD-PARTY EXTENSIONS WILL BE AFFECTED.
 
 IN ORDER TO HELP US IMPROVE THE TRANSITION PROCESS, INCL. DOCUMENTATION AND COMMENTS,
 PLEASE REPORT **ANY** DOUBT, CONFUSION, QUESTIONS, FEEDBACK TO:
     https://github.com/ocornut/imgui/issues/
+If you are using custom widgets, internals or third-party extension that are somehow
+breaking and aren't obvious how to solve, please post in Issues so we can gather
+data and share solutions that may help others.
 
 As part of the plan to reduce impact of API breaking changes, several unfinished
 changes/features/refactors related to font and text systems and scaling will be

imgui.cpp

@@ -5301,8 +5301,12 @@ static void ImGui::UpdateTexturesNewFrame()
         }
         else
         {
-            IM_ASSERT(atlas->Builder != NULL && atlas->Builder->FrameCount != -1 && "If you manage font atlases yourself you need to call ImFontAtlasUpdateNewFrame() on it.");
-            IM_ASSERT(atlas->RendererHasTextures == has_textures && "If you manage font atlases yourself make sure ImGuiBackendFlags_RendererHasTextures is set consistently with atlas->RendererHasTextures as specified in the ImFontAtlasUpdateNewFrame() call.");
+            // (1) If you manage font atlases yourself, e.g. create a ImFontAtlas yourself you need to call ImFontAtlasUpdateNewFrame() on it.
+            // Otherwise, calling ImGui::CreateContext() without parameter will create an atlas owned by the context.
+            // (2) If you have multiple font atlases, make sure the 'atlas->RendererHasTextures' as specified in the ImFontAtlasUpdateNewFrame() call matches for that.
+            // (3) If you have multiple imgui contexts, they also need to have a matching value for ImGuiBackendFlags_RendererHasTextures.
+            IM_ASSERT(atlas->Builder != NULL && atlas->Builder->FrameCount != -1);
+            IM_ASSERT(atlas->RendererHasTextures == has_textures);
         }
     }
 }
@@ -16967,7 +16971,7 @@ void ImGui::DebugNodeFont(ImFont* font)
         if (baked->ContainerFont != font)
             continue;
         PushID(baked_n);
-        if (TreeNode("Glyphs", "Baked at { %.2fpx, d.%.1f }: %d glyphs%s", baked->Size, baked->RasterizerDensity, baked->Glyphs.Size, (baked->LastUsedFrame < atlas->Builder->FrameCount - 1) ? " *Unused*" : ""))
+        if (TreeNode("Glyphs", "Baked at { %.2fpx, d.%.2f }: %d glyphs%s", baked->Size, baked->RasterizerDensity, baked->Glyphs.Size, (baked->LastUsedFrame < atlas->Builder->FrameCount - 1) ? " *Unused*" : ""))
         {
             if (SmallButton("Load all"))
                 for (unsigned int base = 0; base <= IM_UNICODE_CODEPOINT_MAX; base++)