Fonts: fixed an issue when a font using MergeMode has a reference size specified but the target font doesn't.

docs/CHANGELOG.txt

@@ -46,6 +46,10 @@ Breaking Changes:
 
 Other Changes:
 
+- Fonts: fixed an issue when a font using MergeMode has a reference size
+  specified but the target font doesn't. Usually either all fonts should
+  have a reference size (only required when specifying e.g. GlyphOffset),
+  or none should have a reference size. 
 - Windows: fixed an issue where resizable child windows would emit border
   logic when hidden/non-visible (e.g. when in a docked window that is not
   selected), impacting code not checking for BeginChild() return value. (#8815)

imgui_draw.cpp

@@ -4573,15 +4573,16 @@ static bool ImGui_ImplStbTrueType_FontSrcInit(ImFontAtlas* atlas, ImFontConfig*
     }
     src->FontLoaderData = bd_font_data;
 
+    const float ref_size = src->DstFont->Sources[0]->SizePixels;
     if (src->MergeMode && src->SizePixels == 0.0f)
-        src->SizePixels = src->DstFont->Sources[0]->SizePixels;
+        src->SizePixels = ref_size;
 
     if (src->SizePixels >= 0.0f)
         bd_font_data->ScaleFactor = stbtt_ScaleForPixelHeight(&bd_font_data->FontInfo, 1.0f);
     else
         bd_font_data->ScaleFactor = stbtt_ScaleForMappingEmToPixels(&bd_font_data->FontInfo, 1.0f);
-    if (src->MergeMode && src->SizePixels != 0.0f)
-        bd_font_data->ScaleFactor *= src->SizePixels / src->DstFont->Sources[0]->SizePixels; // FIXME-NEWATLAS: Should tidy up that a bit
+    if (src->MergeMode && src->SizePixels != 0.0f && ref_size != 0.0f)
+        bd_font_data->ScaleFactor *= src->SizePixels / ref_size; // FIXME-NEWATLAS: Should tidy up that a bit
 
     return true;
 }