Created attachment 349593 [details]
attachment showing a potential fix (in function gst_qt_mux_start_file(...) under .../gst/isomp4/gstqtmux.c

There should be no seek in the mux when we set property of "fragment-duration" to non-zero and "streamable" to TRUE. However I found the qtmux still did "seek" under the function gst_qt_mux_stop_file(...) for updating the qtmux->moov->mvex.mehd.fragment_duration in our case.

Basically the "streamable" config got ignored because the qtmux_klass->format != GST_QT_MUX_FORMAT_ISML in our case. The streamable was set in a later stage  by the following piece of code under function gst_qt_mux_start_file (GstQTMux * qtmux) when the code found that the downstream is not seekable:
======
    case GST_QT_MUX_MODE_FRAGMENTED:
      if (!gst_qt_mux_downstream_is_seekable (qtmux)) {
        GST_WARNING_OBJECT (qtmux, "downstream is not seekable, but "
            "streamable=false. Will ignore that and create streamable output "
            "instead");
        qtmux->streamable = TRUE;
        g_object_notify (G_OBJECT (qtmux), "streamable");
======

However it missed to update the qtmux->mux_mode to GST_QT_MUX_MODE_FRAGMENTED_STREAMABLE from GST_QT_MUX_MODE_FRAGMENTED after setting qtmux->streamable = TRUE

As we are not in GST_QT_MUX_MODE_FRAGMENTED_STREAMABLE which is needed to prevent the "seek" described above, we hit the issue.

A potential fix tested in our case is attached (screenshot of the change).

Please let us know if you need any more information regarding this bug.

Thanks,

Song