Don't merge ImDrawCmd which have had their IdxOffset changed to not be sequential. Fixed CTRL+Tab into an empty window causing artefacts on the highlight rectangle due to bad reordering on ImDrawCmd.

This is bit of a weird edge case adding weight to ImDrawCmd merging, if we could rework the mess in RenderDimmedBackgroundBehindWindow() we may be able to undo some of that.

docs/CHANGELOG.txt

@@ -107,6 +107,7 @@ Other Changes:
   handled by core automatically for all kind of inputs. (#4858, #2787, #1992, #3383, #2525, #1320)
   - New IO functions for keyboard/gamepad: AddKeyEvent(), AddKeyAnalogEvent(), AddKeyModsEvent().
   - New IO functions for mouse: AddMousePosEvent(), AddMouseButtonEvent(), AddMouseWheelEvent().
+- Fixed CTRL+Tab into an empty window causing artefacts on the highlight rectangle due to bad reordering on ImDrawCmd.
 - Fixed a situation where CTRL+Tab or Modal can occasionally lead to the creation of ImDrawCmd with zero triangles,
   which would makes the draw operation of some backends assert (e.g. Metal with debugging). (#4857)
 - Popups: Fixed a regression crash when a new window is created after a modal on the same frame. (#4920) [@rokups]

imgui.cpp

@@ -4678,6 +4678,7 @@ static void ImGui::RenderDimmedBackgroundBehindWindow(ImGuiWindow* window, ImU32
     {
         // We've already called AddWindowToDrawData() which called DrawList->ChannelsMerge() on DockNodeHost windows,
         // and draw list have been trimmed already, hence the explicit recreation of a draw command if missing.
+        // FIXME: This is creating complication, might be simpler if we could inject a drawlist in drawdata at a given position and not attempt to manipulate ImDrawCmd order.
         ImDrawList* draw_list = window->RootWindow->DrawList;
         if (draw_list->CmdBuffer.Size == 0)
             draw_list->AddDrawCmd();
@@ -4688,7 +4689,7 @@ static void ImGui::RenderDimmedBackgroundBehindWindow(ImGuiWindow* window, ImU32
         draw_list->CmdBuffer.pop_back();
         draw_list->CmdBuffer.push_front(cmd);
         draw_list->PopClipRect();
-        draw_list->_PopUnusedDrawCmd(); // Since are past the calls to AddDrawListToDrawData() we don't have a _PopUnusedDrawCmd() running on commands.
+        draw_list->AddDrawCmd(); // We need to create a command as CmdBuffer.back().IdxOffset won't be correct if we append to same command.
     }
 }
 
@@ -4713,6 +4714,8 @@ static void ImGui::RenderDimmedBackgrounds()
 {
     ImGuiContext& g = *GImGui;
     ImGuiWindow* modal_window = GetTopMostAndVisiblePopupModal();
+    if (g.DimBgRatio <= 0.0f && g.NavWindowingHighlightAlpha <= 0.0f)
+        return;
     const bool dim_bg_for_modal = (modal_window != NULL);
     const bool dim_bg_for_window_list = (g.NavWindowingTargetAnim != NULL && g.NavWindowingTargetAnim->Active);
     if (!dim_bg_for_modal && !dim_bg_for_window_list)

imgui_draw.cpp

@@ -491,6 +491,7 @@ void ImDrawList::AddCallback(ImDrawCallback callback, void* callback_data)
 #define ImDrawCmd_HeaderSize                            (IM_OFFSETOF(ImDrawCmd, VtxOffset) + sizeof(unsigned int))
 #define ImDrawCmd_HeaderCompare(CMD_LHS, CMD_RHS)       (memcmp(CMD_LHS, CMD_RHS, ImDrawCmd_HeaderSize))    // Compare ClipRect, TextureId, VtxOffset
 #define ImDrawCmd_HeaderCopy(CMD_DST, CMD_SRC)          (memcpy(CMD_DST, CMD_SRC, ImDrawCmd_HeaderSize))    // Copy ClipRect, TextureId, VtxOffset
+#define ImDrawCmd_AreSequentialIdxOffset(CMD_0, CMD_1)  (CMD_0->IdxOffset + CMD_0->ElemCount == CMD_1->IdxOffset)
 
 // Try to merge two last draw commands
 void ImDrawList::_TryMergeDrawCmds()
@@ -498,7 +499,7 @@ void ImDrawList::_TryMergeDrawCmds()
     IM_ASSERT_PARANOID(CmdBuffer.Size > 0);
     ImDrawCmd* curr_cmd = &CmdBuffer.Data[CmdBuffer.Size - 1];
     ImDrawCmd* prev_cmd = curr_cmd - 1;
-    if (ImDrawCmd_HeaderCompare(curr_cmd, prev_cmd) == 0 && curr_cmd->UserCallback == NULL && prev_cmd->UserCallback == NULL)
+    if (ImDrawCmd_HeaderCompare(curr_cmd, prev_cmd) == 0 && ImDrawCmd_AreSequentialIdxOffset(prev_cmd, curr_cmd) && curr_cmd->UserCallback == NULL && prev_cmd->UserCallback == NULL)
     {
         prev_cmd->ElemCount += curr_cmd->ElemCount;
         CmdBuffer.pop_back();
@@ -521,7 +522,7 @@ void ImDrawList::_OnChangedClipRect()
 
     // Try to merge with previous command if it matches, else use current command
     ImDrawCmd* prev_cmd = curr_cmd - 1;
-    if (curr_cmd->ElemCount == 0 && CmdBuffer.Size > 1 && ImDrawCmd_HeaderCompare(&_CmdHeader, prev_cmd) == 0 && prev_cmd->UserCallback == NULL)
+    if (curr_cmd->ElemCount == 0 && CmdBuffer.Size > 1 && ImDrawCmd_HeaderCompare(&_CmdHeader, prev_cmd) == 0 && ImDrawCmd_AreSequentialIdxOffset(prev_cmd, curr_cmd) && prev_cmd->UserCallback == NULL)
     {
         CmdBuffer.pop_back();
         return;
@@ -544,7 +545,7 @@ void ImDrawList::_OnChangedTextureID()
 
     // Try to merge with previous command if it matches, else use current command
     ImDrawCmd* prev_cmd = curr_cmd - 1;
-    if (curr_cmd->ElemCount == 0 && CmdBuffer.Size > 1 && ImDrawCmd_HeaderCompare(&_CmdHeader, prev_cmd) == 0 && prev_cmd->UserCallback == NULL)
+    if (curr_cmd->ElemCount == 0 && CmdBuffer.Size > 1 && ImDrawCmd_HeaderCompare(&_CmdHeader, prev_cmd) == 0 && ImDrawCmd_AreSequentialIdxOffset(prev_cmd, curr_cmd) && prev_cmd->UserCallback == NULL)
     {
         CmdBuffer.pop_back();
         return;
@@ -1738,6 +1739,8 @@ void ImDrawListSplitter::Merge(ImDrawList* draw_list)
 
         if (ch._CmdBuffer.Size > 0 && last_cmd != NULL)
         {
+            // Do not include ImDrawCmd_AreSequentialIdxOffset() in the compare as we rebuild IdxOffset values ourselves.
+            // Manipulating IdxOffset (e.g. by reordering draw commands like done by RenderDimmedBackgroundBehindWindow()) is not supported within a splitter.
             ImDrawCmd* next_cmd = &ch._CmdBuffer[0];
             if (ImDrawCmd_HeaderCompare(last_cmd, next_cmd) == 0 && last_cmd->UserCallback == NULL && next_cmd->UserCallback == NULL)
             {