When in Fullscreen playback mode, toggle play/pause with the hardware Space button.

app/src/main/java/org/schabi/newpipe/fragments/detail/VideoDetailFragment.java

@@ -1891,8 +1891,10 @@ public final class VideoDetailFragment
 
         if (fullscreen) {
             hideSystemUiIfNeeded();
+            viewPager.setVisibility(View.GONE);
         } else {
             showSystemUi();
+            viewPager.setVisibility(View.VISIBLE);
         }
 
         if (relatedStreamsLayout != null) {

app/src/main/java/org/schabi/newpipe/player/VideoPlayerImpl.java

@@ -505,6 +505,11 @@ public class VideoPlayerImpl extends VideoPlayer
         switch (keyCode) {
             default:
                 break;
+            case KeyEvent.KEYCODE_SPACE:
+                if (isFullscreen) {
+                    onPlayPause();
+                }
+                break;
             case KeyEvent.KEYCODE_BACK:
                 if (DeviceUtils.isTv(service) && isControlsVisible()) {
                     hideControls(0, 0);