Fix for ImDrawListSplitter::Merge - corrupted data in buffer (64k+ vertices) with 16-bits indices #3129

imgui.h

@@ -2068,6 +2068,7 @@ struct ImDrawList
     inline    void  PrimVtx(const ImVec2& pos, const ImVec2& uv, ImU32 col)     { PrimWriteIdx((ImDrawIdx)_VtxCurrentIdx); PrimWriteVtx(pos, uv, col); }
     IMGUI_API void  UpdateClipRect();
     IMGUI_API void  UpdateTextureID();
+    IMGUI_API void  UpdateStaleDrawCmd();
 };
 
 // All draw data to render a Dear ImGui frame

imgui_draw.cpp

@@ -461,7 +461,7 @@ void ImDrawList::UpdateClipRect()
 
     // Try to merge with previous command if it matches, else use current command
     ImDrawCmd* prev_cmd = CmdBuffer.Size > 1 ? curr_cmd - 1 : NULL;
-    if (curr_cmd->ElemCount == 0 && prev_cmd && memcmp(&prev_cmd->ClipRect, &curr_clip_rect, sizeof(ImVec4)) == 0 && prev_cmd->TextureId == GetCurrentTextureId() && prev_cmd->UserCallback == NULL)
+    if (curr_cmd->ElemCount == 0 && prev_cmd && memcmp(&prev_cmd->ClipRect, &curr_clip_rect, sizeof(ImVec4)) == 0 && prev_cmd->VtxOffset == _VtxCurrentOffset && prev_cmd->TextureId == GetCurrentTextureId() && prev_cmd->UserCallback == NULL)
         CmdBuffer.pop_back();
     else
         curr_cmd->ClipRect = curr_clip_rect;
@@ -480,12 +480,49 @@ void ImDrawList::UpdateTextureID()
 
     // Try to merge with previous command if it matches, else use current command
     ImDrawCmd* prev_cmd = CmdBuffer.Size > 1 ? curr_cmd - 1 : NULL;
-    if (curr_cmd->ElemCount == 0 && prev_cmd && prev_cmd->TextureId == curr_texture_id && memcmp(&prev_cmd->ClipRect, &GetCurrentClipRect(), sizeof(ImVec4)) == 0 && prev_cmd->UserCallback == NULL)
+    if (curr_cmd->ElemCount == 0 && prev_cmd && prev_cmd->TextureId == curr_texture_id && memcmp(&prev_cmd->ClipRect, &GetCurrentClipRect(), sizeof(ImVec4)) == 0 && prev_cmd->VtxOffset == _VtxCurrentOffset && prev_cmd->UserCallback == NULL)
         CmdBuffer.pop_back();
     else
         curr_cmd->TextureId = curr_texture_id;
 }
 
+// Use this when you're about to draw to pre-cached ImDrawCmd.
+// It act like UpdateClipRect, UpdateTextureID and (not implemented UpdateVtxOffset) combined.
+void ImDrawList::UpdateStaleDrawCmd()
+{
+    // If current command is used with different settings we need to add a new command
+    ImDrawCmd* curr_cmd = CmdBuffer.Size ? &CmdBuffer.back() : NULL;
+    const ImVec4 curr_clip_rect = GetCurrentClipRect();
+    const ImTextureID curr_texture_id = GetCurrentTextureId();
+    if (!curr_cmd || (curr_cmd->ElemCount != 0 && ((curr_cmd->VtxOffset != _VtxCurrentOffset) || (curr_cmd->TextureId != curr_texture_id) || (memcmp(&curr_cmd->ClipRect, &curr_clip_rect, sizeof(ImVec4)) != 0))) || curr_cmd->UserCallback != NULL)
+    {
+        AddDrawCmd();
+        return;
+    }
+
+    // Try to merge with previous command if it matches, else use current command
+    ImDrawCmd* prev_cmd = CmdBuffer.Size > 1 ? curr_cmd - 1 : NULL;
+    const bool can_try_merge = (curr_cmd->ElemCount == 0 && prev_cmd && prev_cmd->UserCallback != NULL);
+    const bool has_same_texture_id = can_try_merge && (prev_cmd->TextureId == curr_texture_id);
+    const bool has_same_clip_rect  = can_try_merge && (memcmp(&prev_cmd->ClipRect, &curr_clip_rect, sizeof(ImVec4)) == 0);
+    const bool has_same_vtx_offset = can_try_merge && (prev_cmd->VtxOffset == _VtxCurrentOffset);
+
+    if (has_same_texture_id && has_same_clip_rect && has_same_vtx_offset)
+    {
+        CmdBuffer.pop_back();
+        return;
+    }
+
+    if (!has_same_texture_id)
+        curr_cmd->TextureId = curr_texture_id;
+
+    if (!has_same_clip_rect)
+        curr_cmd->ClipRect = curr_clip_rect;
+
+    if (!has_same_vtx_offset)
+        curr_cmd->VtxOffset = _VtxCurrentOffset;
+}
+
 #undef GetCurrentClipRect
 #undef GetCurrentTextureId
 
@@ -1403,8 +1440,7 @@ void ImDrawListSplitter::Merge(ImDrawList* draw_list)
         if (int sz = ch._IdxBuffer.Size) { memcpy(idx_write, ch._IdxBuffer.Data, sz * sizeof(ImDrawIdx)); idx_write += sz; }
     }
     draw_list->_IdxWritePtr = idx_write;
-    draw_list->UpdateClipRect(); // We call this instead of AddDrawCmd(), so that empty channels won't produce an extra draw call.
-    draw_list->UpdateTextureID();
+    draw_list->UpdateStaleDrawCmd(); // We call this instead of AddDrawCmd(), so that empty channels won't produce an extra draw call.
     _Count = 1;
 }
 
@@ -1420,6 +1456,7 @@ void ImDrawListSplitter::SetCurrentChannel(ImDrawList* draw_list, int idx)
     memcpy(&draw_list->CmdBuffer, &_Channels.Data[idx]._CmdBuffer, sizeof(draw_list->CmdBuffer));
     memcpy(&draw_list->IdxBuffer, &_Channels.Data[idx]._IdxBuffer, sizeof(draw_list->IdxBuffer));
     draw_list->_IdxWritePtr = draw_list->IdxBuffer.Data + draw_list->IdxBuffer.Size;
+    draw_list->UpdateStaleDrawCmd();
 }
 
 //-----------------------------------------------------------------------------

imgui_widgets.cpp

@@ -7553,7 +7553,7 @@ void ImGui::PushColumnsBackground()
     int cmd_size = window->DrawList->CmdBuffer.Size;
     PushClipRect(columns->HostClipRect.Min, columns->HostClipRect.Max, false);
     IM_UNUSED(cmd_size);
-    IM_ASSERT(cmd_size == window->DrawList->CmdBuffer.Size); // Being in channel 0 this should not have created an ImDrawCmd
+    IM_ASSERT(cmd_size >= window->DrawList->CmdBuffer.Size); // Being in channel 0 this should not have created an ImDrawCmd
 }
 
 void ImGui::PopColumnsBackground()