Columns: Clipping columns borders on Y axis on CPU because some GPU drivers appears to be unhappy with triangle spanning large regions (not sure why tbh). (#125)

Demo: Columns: Adding a billion of extra lines and using clipper.

imgui.cpp

@@ -10511,9 +10511,10 @@ void ImGui::EndColumns()
             }
 
             // Draw column
+            // We clip the Y boundaries CPU side because very long triangles are mishandled by some GPU drivers.
             const ImU32 col = GetColorU32(held ? ImGuiCol_SeparatorActive : hovered ? ImGuiCol_SeparatorHovered : ImGuiCol_Separator);
             const float xi = (float)(int)x;
-            window->DrawList->AddLine(ImVec2(xi, y1 + 1.0f), ImVec2(xi, y2), col);
+            window->DrawList->AddLine(ImVec2(xi, ImMax(y1 + 1.0f, window->ClipRect.Min.y)), ImVec2(xi, ImMin(y2, window->ClipRect.Max.y)), col);
         }
 
         // Apply dragging after drawing the column lines, so our rendered lines are in sync with how items were displayed during the frame.

imgui_demo.cpp

@@ -1652,14 +1652,19 @@ void ImGui::ShowTestWindow(bool* p_open)
         if (ImGui::TreeNode("Horizontal Scrolling"))
         {
             ImGui::SetNextWindowContentWidth(1500);
-            ImGui::BeginChild("##scrollingregion", ImVec2(0, 120), false, ImGuiWindowFlags_HorizontalScrollbar);
+            ImGui::BeginChild("##ScrollingRegion", ImVec2(0, ImGui::GetFontSize() * 20), false, ImGuiWindowFlags_HorizontalScrollbar);
             ImGui::Columns(10);
-            for (int i = 0; i < 20; i++)
+            int ITEMS_COUNT = 2000;
+            ImGuiListClipper clipper(ITEMS_COUNT);  // Also demonstrate using the clipper for large list
+            while (clipper.Step())
+            {
+                for (int i = clipper.DisplayStart; i < clipper.DisplayEnd; i++)
                     for (int j = 0; j < 10; j++)
                     {
                         ImGui::Text("Line %d Column %d...", i, j);
                         ImGui::NextColumn();
                     }
+            }
             ImGui::Columns(1);
             ImGui::EndChild();
             ImGui::TreePop();