Multi-Select: Box-Select+Tables: fixed using BeginMultiSelect() before table layout is locked. (#8250)

docs/CHANGELOG.txt

@@ -72,6 +72,8 @@ Other Changes:
     and ImGuiSelectionUserData is technically opaque storage. (#7994, #1861)
     (we will probably bring this back as a minor optimization if we have a way to for
     user to tell us ImGuiSelectionUserData are indices)
+  - Box-Select, Tables: fixed an issue when calling `BeginMultiSelect()` in a table
+    before layout has been locked (first row or headers row submitted). (#8250)
 - Fonts:
   - imgui_freetype: add FreeType headers & compiled version in 'About Dear ImGui' details.
 - Clipper:

imgui_tables.cpp

@@ -1647,7 +1647,7 @@ void ImGui::TableSetupColumn(const char* label, ImGuiTableColumnFlags flags, flo
     ImGuiTable* table = g.CurrentTable;
     IM_ASSERT_USER_ERROR_RET(table != NULL, "Call should only be done while in BeginTable() scope!");
     IM_ASSERT_USER_ERROR_RET(table->DeclColumnsCount < table->ColumnsCount, "TableSetupColumn(): called too many times!");
-    IM_ASSERT_USER_ERROR_RET(table->IsLayoutLocked == false, "TableSetupColumn(): need to call before first row!");
+    IM_ASSERT_USER_ERROR_RET(table->IsLayoutLocked == false, "TableSetupColumn(): need to call before first row!"); // Table layout is locked when submitting a row or when calling BeginMultiSelect() with box-select.
     IM_ASSERT((flags & ImGuiTableColumnFlags_StatusMask_) == 0 && "Illegal to pass StatusMask values to TableSetupColumn()");
 
     ImGuiTableColumn* column = &table->Columns[table->DeclColumnsCount];

imgui_widgets.cpp

@@ -7958,8 +7958,12 @@ ImGuiMultiSelectIO* ImGui::BeginMultiSelect(ImGuiMultiSelectFlags flags, int sel
     // FIXME: Workaround to the fact we override CursorMaxPos, meaning size measurement are lost. (#8250)
     // They should perhaps be stacked properly?
     if (ImGuiTable* table = g.CurrentTable)
-        if (table->CurrentColumn != -1)
+    {
+        if (!table->IsLayoutLocked)
+            TableUpdateLayout(table);
+        else if (table->CurrentColumn != -1)
             TableEndCell(table); // This is currently safe to call multiple time. If that properly is lost we can extract the "save measurement" part of it.
+    }
 
     // FIXME: BeginFocusScope()
     const ImGuiID id = window->IDStack.back();