OS X 10.9 Fullscreen on second display doesn't stretch to full screen, but only part of it

I'm on OS X 10.9 and with this OS upgrade, the ability to run fullscreen apps on a second display while working on another was implemented.

What I want:
Play YouTube and other online sources (divxstage.eu to name one) videos on my second (1080p) screen while working on my internal MacBook Pro display.

What happens:

Case (1): Youtube videos can be run in fullscreen mode, so if I click on the button it expands to the whole secondary display. When switching back to my MBP, after a while OR (when in a playlist) upon starting the next video it resizes to only a portion (I believe about the same size of the MBP display!) of the screen in the upper left corner, however cutting off the video and displaying a 3/4 black screen. (Example: if there was a circle in the video, you'd now see like a segment of this circle)

Case (2): I found that with e.g. divxstage.eu, videos do not even expand to the full screen, but leave it blank in 3/4 right away. (Same size as with YouTube videos)

Has anyone experienced this, too? Can anybody help, a possible solution, workaround, or even just the source of the problem?

Solution 1:

Here's my workaround: right click on the 1/4-screen video and, if you're using Flash 15.0.0.xxx, you should see a Stretching is <type> item in the options list. Click on that and the video will fill the screen. Keep repeating until you get back to the 'Stretching is none' setting, unless you prefer something else.

I have the same issue when using firstrowsports.eu on my 2014 MBP, multiple different browsers, no plugins, no additional screen, hardware acceleration on and off, Mavericks and now Yosemite.