Worked on the maths/moodle-qtype_stack repository to enhance the reliability of dropdown input handling by addressing a parsing robustness bug. Focused on backend development using PHP, the work involved resolving function call type errors and argument-passing issues within the stack_dropdown_input component. Introduced a new mechanism, castext2_placeholder_holder, to manage placeholders during post-processing, ensuring accurate parsing and display of dropdown answers. The changes improved type safety and interface alignment, reducing edge-case failures and synchronizing input with display values. Demonstrated careful debugging and change management, resulting in a more stable dropdown question experience without introducing new features during the development period.