With a browser extension, you can disable YouTube's Dash Playback to buffer full videos instead of segment by segment.
You may have noticed that when you are playing a YouTube video, it buffers only so much ahead of your current position. And should you pause the video, buffering stops altogether. The reason behind this lackluster form of buffering? YouTube's embrace of Dynamic Adaptive Streaming over HTTP (Dash), which breaks a video into segments, buffering each segment as you make your way through the video. In theory, Dash allows YouTube to adapt to changing network conditions without stalling out at times.
In the real world, Dash means you can't pause a video and come back to it later with it fully buffered, which allows you to skip around the video without waiting for it to rebuffer each time you jump to a new spot.
YouTube doesn't let you disable Dash playback, but browser extension YouTube Center does. It's available for Chrome, Firefox, IE, Opera, and Safari. It has not been approved in the Chrome Web Store, so you'll need to download it to your PC and drag the file into Chrome's extension page. Also, Chrome users will need to check the box on the extension page to enable Developer mode.
YouTube Center installs a small settings button in the upper-right corner of YouTube. Click on the gear icon to open the settings panel, which conveniently opens over your current YouTube page. It lets you tweak oodles of settings, but for the purposes of this post, you'll need to click on the Player tab at the top and then uncheck the box for Dash Playback. No need to save; just click the X in the upper-right corner to close the settings window and return to your YouTubing ways.
Now, you'll notice that once you begin a video, YouTube will buffer the entire thing, even if you leave it paused and head to another tab. I tried YouTube Center on a Mac with both Chrome and Firefox, and the extension worked seamlessly. I also found it made my default playback window larger, which I didn't always enjoy, since it made browsing related videos while half-watching the current video a more difficult task. Thankfully, on the Resize tab in the settings window, there are many options from which to choose.
Windows users, take note: my fellow How To blogger Nicole Cozma tested YouTube Center with both Chrome and Firefox on a Windows machine and the extension did not work. To conclude, your mileage may vary.