Start portal encoder deadlines after the first permitted capture frame

tools/chatgui/src/stream_publish_engine.cpp

@@ -131,6 +131,7 @@ bool StreamPublishEngine::openSource(const StreamPublishConfig& config) {
             m->backendAnchor = now;
             DEBUG_WARN(DEBUG_CATEGORY_VIDEO, "desktop capture has no source PTS; using monotonic callback estimate");
         }
+        if (m->framePts < 0 && m->wanted.source.id == "portal") m->started = monotonicUs();
         m->frame = m->previewFrame = frame; m->framePts = std::max<int64_t>(1, now - m->origin); m->frameGeneration = m->generation;
         m->wake.notify_all();
     }, Qt::DirectConnection);