This has preoccupied me for an hour or two but happily has finally righted itself. No idea how or why it happened or went away. So I'll describe the symptoms in case
1. Anyone else observes the same sometime.
2. It's relevant to any other issue, or offers clues/insights.
To follow along, do this in a fresh project:
Place an object in track 1. Then Video effects > Chroma Key > Click drop-down arrow top right > Load effects mask > Form1_1920x1080.jpg (the first in the folder, but any of them will do). That places the mask in track 2. Then choose White for the Chromakey setting of the mask.
Depending on which object I selected, (track 1 object, track 2 mask, none or both) the playback of that section was either correct (a view of the object through the rectangular mask) or incorrect (white rectangle mask over the object). I could only get correct display when the mask was selected. And even then it wasn't immediate, I had to move the marker first.
Here's a couple of screnshots (using the last mask, Oval9):
I loaded MEP17 instead and that had no such problem. Playback was independent of whatever track was selected, as it should be.
I reloaded MEP 2013 and the problem was still present on that pair of clips. Then I chose another object in track 1 and dragged my own mask onto track 2 below it. That did not cause the problem. I returned to the problem pair - and they were now OK!
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Terry, East Grinstead, UK