SDK: Premiere fails to call into third-party plugins with PF_Cmd_USER_CHANGED_PARAM for sliders

When users change the value of a slider parameter declared by a plugin with the

PF_ParamFlag_SUPERVISE flag, Premiere fails to notify the plugin via PF_Cmd_USER_CHANGED_PARAM.

Steps to reproduce:

- Take an existing plug-in based on the AE SDK, and make one of its slider parameters observable by setting its PF_ParamFlag_SUPERVISE flag.

- Set a breakpoint / print something to the console / beep when handling the PF_Cmd_USER_CHANGED_PARAM command.

- Apply the plugin inside Premiere Pro.

- Change the slider’s value and notice that PF_Cmd_USER_CHANGED_PARAM is never invoked.

Expected:

Plug-ins should receive a PF_Cmd_USER_CHANGED_PARAM command whenever the value assigned to a slider parameter is changed.

Notes:

Sliders can trigger a large number of changes very quickly, and it would be perfectly fine if Premiere were to coalesce multiple changes into one. BUT it is vital that Premiere issue at least one PF_Cmd_USER_CHANGED_PARAM command, when the user is done scrubbing. Ideally, it should issue multiple PF_Cmd_USER_CHANGED_PARAM at a maximum rate (2 per second?) so as to periodically give plugins a chance to respond.