Not all expected cells are selected when drag-selecting in an x-scrolling table

[tombuskens]

Version/Branch of Dear ImGui:

Veversion 1.91.1 (19110), docking branch

Back-ends:

imgui_impl_win32.cpp + imgui_impl_dx12.cpp

Compiler, OS:

Windows 10 + MSVC 14.39.33218

Full config/build information:

Dear ImGui 1.91.1 (19110)
--------------------------------
sizeof(size_t): 8, sizeof(ImDrawIdx): 2, sizeof(ImDrawVert): 20
define: __cplusplus=202002
define: _WIN32
define: _WIN64
define: _MSC_VER=1939
define: _MSVC_LANG=202002
define: IMGUI_HAS_VIEWPORT
define: IMGUI_HAS_DOCK
--------------------------------
io.BackendPlatformName: imgui_impl_win32
io.BackendRendererName: imgui_impl_dx12
io.ConfigFlags: 0x00000481
 NavEnableKeyboard
 DockingEnable
 ViewportsEnable
io.ConfigViewportsNoAutoMerge
io.ConfigViewportsNoDecoration
io.ConfigInputTextCursorBlink
io.ConfigWindowsResizeFromEdges
io.ConfigWindowsMoveFromTitleBarOnly
io.ConfigMemoryCompactTimer = 60.0
io.BackendFlags: 0x00001C0E
 HasMouseCursors
 HasSetMousePos
 PlatformHasViewports
 HasMouseHoveredViewport
 RendererHasVtxOffset
 RendererHasViewports
--------------------------------
io.Fonts: 2 fonts, Flags: 0x00000000, TexSize: 512,64
io.DisplaySize: 784.00,561.00
io.DisplayFramebufferScale: 1.00,1.00
--------------------------------
style.WindowPadding: 8.00,8.00
style.WindowBorderSize: 1.00
style.FramePadding: 4.00,3.00
style.FrameRounding: 0.00
style.FrameBorderSize: 0.00
style.ItemSpacing: 8.00,4.00
style.ItemInnerSpacing: 4.00,4.00

Details:

When drag-selecting in a table with ImGuiTableFlags_ScrollX enabled, and selection set to ImGuiMultiSelectFlags_BoxSelect2d, it seems that not all expected cells are selected.

See the video below; the expected result would be that all cells in the rectangular selection area would be selected, but some of the cells that scroll out of view are not selected.

Screenshots/Video:

multiselect.mp4

Minimal, Complete and Verifiable Example code:

The issue is reproducible with some modified demo code.
The modifications are basically - change to ImGuiMultiSelectFlags_BoxSelect2d / ImGuiTableFlags_ScrollX, add some columns, and track cell selection instead of rows.

The behavior seems to be triggered most easily by scrolling to the right, then drag-selecting from top-right to bottom-left, though generally scrolling while selecting can result in some cells not being selected correctly.

        IMGUI_DEMO_MARKER("Widgets/Selection State/Multi-Select (in a table)");
        if (ImGui::TreeNode("Multi-Select (in a table)"))
        {
            static ImGuiSelectionBasicStorage selection;

            const int ITEMS_COUNT = 10000;
            const int COLUMN_COUNT = 5;
            ImGui::Text("Selection: %d/%d", selection.Size, ITEMS_COUNT);
            if (ImGui::BeginTable("##Basket", COLUMN_COUNT, ImGuiTableFlags_ScrollY | ImGuiTableFlags_ScrollX | ImGuiTableFlags_RowBg | ImGuiTableFlags_BordersOuter, ImVec2(300, 300)))
            {
                ImGui::TableSetupColumn("Object", ImGuiTableColumnFlags_WidthFixed, 100);
                ImGui::TableSetupColumn("One", ImGuiTableColumnFlags_WidthFixed, 100);
                ImGui::TableSetupColumn("Two", ImGuiTableColumnFlags_WidthFixed, 100);
                ImGui::TableSetupColumn("Three", ImGuiTableColumnFlags_WidthFixed, 100);
                ImGui::TableSetupColumn("Four", ImGuiTableColumnFlags_WidthFixed, 100);

                ImGui::TableSetupScrollFreeze(0, 1);
                ImGui::TableHeadersRow();

                ImGuiMultiSelectFlags flags = ImGuiMultiSelectFlags_ClearOnEscape | ImGuiMultiSelectFlags_BoxSelect2d;
                ImGuiMultiSelectIO* ms_io = ImGui::BeginMultiSelect(flags, selection.Size, ITEMS_COUNT * COLUMN_COUNT);
                selection.ApplyRequests(ms_io);

                ImGuiListClipper clipper;
                clipper.Begin(ITEMS_COUNT);
                if (ms_io->RangeSrcItem != -1)
                    clipper.IncludeItemByIndex((int)ms_io->RangeSrcItem / COLUMN_COUNT); // Ensure RangeSrc item is not clipped.
                while (clipper.Step())
                {
                    for (int n = clipper.DisplayStart; n < clipper.DisplayEnd; n++)
                    {
                        ImGui::TableNextRow();
                        ImGui::TableNextColumn();
                        char label[64];
                        sprintf(label, "Object %05d: %s", n, ExampleNames[n % IM_ARRAYSIZE(ExampleNames)]);
                        bool item_is_selected;
                        int idx = n * COLUMN_COUNT;
                        item_is_selected = selection.Contains((ImGuiID)idx);
                        ImGui::SetNextItemSelectionUserData(idx);
                        ImGui::Selectable(label, item_is_selected, ImGuiSelectableFlags_AllowOverlap);

                        ++idx;
                        for (idx; idx < n * COLUMN_COUNT + 5; idx++)
                        {
                            ImGui::TableNextColumn();
                            item_is_selected = selection.Contains((ImGuiID)idx);
                            ImGui::SetNextItemSelectionUserData(idx);
                            ImGui::Selectable(label, item_is_selected, ImGuiSelectableFlags_AllowOverlap);
                        }
                    }
                }

                ms_io = ImGui::EndMultiSelect();
                selection.ApplyRequests(ms_io);
                ImGui::EndTable();
            }
            ImGui::TreePop();
        }