Fonts: Ensure calling AddFontXXX function doesn't invalidates ImFont's ConfigData pointers prior to building again. (#6825)

docs/CHANGELOG.txt

diff --git a/docs/CHANGELOG.txt b/docs/CHANGELOG.txt
index 4bf90ae..ef5de7b 100644
--- a/docs/CHANGELOG.txt
+++ b/docs/CHANGELOG.txt
@@ -82,6 +82,8 @@
   This is because our layout/font system currently doesn't fully support non-integer sizes. Until
   it does, this has been a common pitfall leading to more or less subtle issues. (#3164, #3309, #6800)
 - Fonts: Better assert during load when passing truncated font data or wrong data size. (#6822)
+- Fonts: Ensure calling AddFontXXX function doesn't invalidates ImFont's ConfigData pointers
+  prior to building again. (#6825)
 - InputTextMultiline: Fixed a crash pressing Down on last empty line of a multiline buffer.
   (regression from 1.89.2, only happened in some states). (#6783, #6000)
 - InputTextMultiline: Fixed Tabbing cycle leading to a situation where Enter key wouldn't

imgui_draw.cpp

diff --git a/imgui_draw.cpp b/imgui_draw.cpp
index b920f5e..4192d56 100644
--- a/imgui_draw.cpp
+++ b/imgui_draw.cpp
@@ -2183,6 +2183,8 @@
     if (new_font_cfg.DstFont->EllipsisChar == (ImWchar)-1)
         new_font_cfg.DstFont->EllipsisChar = font_cfg->EllipsisChar;
 
+    ImFontAtlasUpdateConfigDataPointers(this);
+
     // Invalidate texture
     TexReady = false;
     ClearTexData();
@@ -2695,19 +2697,31 @@
 
 #endif // IMGUI_ENABLE_STB_TRUETYPE
 
+void ImFontAtlasUpdateConfigDataPointers(ImFontAtlas* atlas)
+{
+    for (ImFontConfig& font_cfg : atlas->ConfigData)
+    {
+        ImFont* font = font_cfg.DstFont;
+        if (!font_cfg.MergeMode)
+        {
+            font->ConfigData = &font_cfg;
+            font->ConfigDataCount = 0;
+        }
+        font->ConfigDataCount++;
+    }
+}
+
 void ImFontAtlasBuildSetupFont(ImFontAtlas* atlas, ImFont* font, ImFontConfig* font_config, float ascent, float descent)
 {
     if (!font_config->MergeMode)
     {
         font->ClearOutputData();
         font->FontSize = font_config->SizePixels;
-        font->ConfigData = font_config;
-        font->ConfigDataCount = 0;
+        IM_ASSERT(font->ConfigData == font_config);
         font->ContainerAtlas = atlas;
         font->Ascent = ascent;
         font->Descent = descent;
     }
-    font->ConfigDataCount++;
 }
 
 void ImFontAtlasBuildPackCustomRects(ImFontAtlas* atlas, void* stbrp_context_opaque)
@@ -3269,6 +3283,7 @@
         max_codepoint = ImMax(max_codepoint, (int)Glyphs[i].Codepoint);
 
     // Build lookup table
+    IM_ASSERT(Glyphs.Size > 0 && "Font has not loaded glyph!");
     IM_ASSERT(Glyphs.Size < 0xFFFF); // -1 is reserved
     IndexAdvanceX.clear();
     IndexLookup.clear();

imgui_internal.h

diff --git a/imgui_internal.h b/imgui_internal.h
index b153cc1..61fc4ba 100644
--- a/imgui_internal.h
+++ b/imgui_internal.h
@@ -3385,6 +3385,7 @@
 #ifdef IMGUI_ENABLE_STB_TRUETYPE
 IMGUI_API const ImFontBuilderIO* ImFontAtlasGetBuilderForStbTruetype();
 #endif
+IMGUI_API void      ImFontAtlasUpdateConfigDataPointers(ImFontAtlas* atlas);
 IMGUI_API void      ImFontAtlasBuildInit(ImFontAtlas* atlas);
 IMGUI_API void      ImFontAtlasBuildSetupFont(ImFontAtlas* atlas, ImFont* font, ImFontConfig* font_config, float ascent, float descent);
 IMGUI_API void      ImFontAtlasBuildPackCustomRects(ImFontAtlas* atlas, void* stbrp_context_opaque);