Description Pierre Ossman 2018-09-20 16:17:53 CEST

I am seeing delays in xfce4-terminal, and some times it doesn't update at all until I do some more action (e.g. move the mouse).

After some digging it turns out that this is because of some bad interaction between the Composite extension and the VNC code:

Some windows can be implicitly composited. Generally this is because the window was created with a visual that is incompatible with the root window (the "screen"). So Composite steps in to automatically convert and update things whenever the contents of the window changes.

The way it does this is to register a "block handler" that can do the conversion an updating. A block handler is a mechanism used to prepare which file descriptors you want monitored, and the timeout you want for select()/poll().

The reason it breaks is because Composite's block handler might be run after VNC's block handler. So VNC ran first and didn't see any work to do so it set an infinite timeout. After that Composite ran and updated the display. VNC noticed this and set up a timer to update the client(s). However its block handler had already executed so it newer got a chance to tell the X core that it wanted a short timeout.

The end result is that the X server went to sleep until something external made it wake up. At that point VNC got a chance to run again and push the updates to the client(s).