From 64d8d302fb379dd9d7fbe5475a32bd6d61004db5 Mon Sep 17 00:00:00 2001
From: ocornut <omarcornut@gmail.com>
Date: Wed, 10 Jun 2020 19:16:06 +0200
Subject: [PATCH] ImDrawList: Fixed VtxOffset change leading to unnecessary
 leading empty ImDrawCmd in certain cases.

---
 imgui.h        |  1 +
 imgui_draw.cpp | 18 ++++++++++++++++--
 2 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/imgui.h b/imgui.h
index 3f25e81e..4de17899 100644
--- a/imgui.h
+++ b/imgui.h
@@ -2075,6 +2075,7 @@ struct ImDrawList
     IMGUI_API void  _PopUnusedDrawCmd();
     IMGUI_API void  _OnChangedClipRect();
     IMGUI_API void  _OnChangedTextureID();
+    IMGUI_API void  _OnChangedVtxOffset();
 };
 
 // All draw data to render a Dear ImGui frame
diff --git a/imgui_draw.cpp b/imgui_draw.cpp
index 94f223ba..5b0a9855 100644
--- a/imgui_draw.cpp
+++ b/imgui_draw.cpp
@@ -514,6 +514,21 @@ void ImDrawList::_OnChangedTextureID()
     curr_cmd->TextureId = _CmdHeader.TextureId;
 }
 
+void ImDrawList::_OnChangedVtxOffset()
+{
+    // We don't need to compare curr_cmd->VtxOffset != _CmdHeader.VtxOffset because we know it'll be different at the time we call this.
+    _VtxCurrentIdx = 0;
+    ImDrawCmd* curr_cmd = &CmdBuffer.Data[CmdBuffer.Size - 1];
+    IM_ASSERT(curr_cmd->VtxOffset != _CmdHeader.VtxOffset);
+    if (curr_cmd->ElemCount != 0)
+    {
+        AddDrawCmd();
+        return;
+    }
+    IM_ASSERT(curr_cmd->UserCallback == NULL);
+    curr_cmd->VtxOffset = _CmdHeader.VtxOffset;
+}
+
 // Render-level scissoring. This is passed down to your render function but not used for CPU-side coarse clipping. Prefer using higher-level ImGui::PushClipRect() to affect logic (hit-testing and widget culling)
 void ImDrawList::PushClipRect(ImVec2 cr_min, ImVec2 cr_max, bool intersect_with_current_clip_rect)
 {
@@ -570,8 +585,7 @@ void ImDrawList::PrimReserve(int idx_count, int vtx_count)
     if (sizeof(ImDrawIdx) == 2 && (_VtxCurrentIdx + vtx_count >= (1 << 16)) && (Flags & ImDrawListFlags_AllowVtxOffset))
     {
         _CmdHeader.VtxOffset = VtxBuffer.Size;
-        _VtxCurrentIdx = 0;
-        AddDrawCmd();
+        _OnChangedVtxOffset();
     }
 
     ImDrawCmd* draw_cmd = &CmdBuffer.Data[CmdBuffer.Size - 1];