The "Proxy/Timecode" check box doesn't have any effect - if the preview pane is set to a proxy size and there are proxy files present, they are used.
Not a massive problem but annoying if you're using a low-quality proxy (to save space) but want to turn it off to see the original.
2.78; master with hash: 69dbeec;
Steps to reproduce:
- Add a video strip to the VSE
- Tick the "Proxy/Timecode" box, set JPEG quality very low e.g. 20 (to make it easy to see the difference) click the "rebuild proxy" button to create the default 25% proxy AVI, let it finish
- Set preview to 25% proxy
- Seek around, observe that the proxy is used by seeing JPEG artifacts
- Untick the "Proxy/Timecode" box, seek around some more - proxy is still used