use bash’s PROMPT_COMMAND as array
Previously, PROMPT_COMMAND was simply overwritten possibly breaking anything already added to it. Bash may use PROMPT_COMMAND as an array. By adding the VTE command via an array compound assignment with the += operator, a PROMPT_COMMAND that has already been set as a (plain) parameter would be converted to an indexed array and appended. This would not work with an associative array (but fail gracefully), but at least in current versions of bash (5.1.8 as of now), these cannot be used properly for PROMPT_COMMAND anyway.