fix(ff): use fixed inputFPS instead of vfr to prevent dropped f… (#6368)

* fix(ff): use fixed inputFPS instead of vfr to prevent dropped frames

packages/server/lib/video_capture.js

@@ -167,10 +167,19 @@ module.exports = {
         if (options.webmInput) {
           cmd
           .inputFormat('webm')
-          .outputOption('-vsync vfr')
+
+          // assume 18 fps. This number comes from manual measurement of avg fps coming from firefox.
+          // TODO: replace this with the 'vfr' option below when dropped frames issue is fixed.
+          .inputFPS(18)
+
+          // 'vsync vfr' (variable framerate) works perfectly but fails on top page navigation
+          // since video timestamp resets to 0, timestamps already written will be dropped
+          // .outputOption('-vsync vfr')
+
           // this is to prevent the error "invalid data input" error
           // when input frames have an odd resolution
           .videoFilters(`crop='floor(in_w/2)*2:floor(in_h/2)*2'`)
+
           // same as above but scales instead of crops
           // .videoFilters("scale=trunc(iw/2)*2:trunc(ih/2)*2")
         } else {