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

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();