The Mask popup menu can be overwhelming upon first encounter. However, if you follow the next
few steps you can create a single simple mask without having to understand every possible parameter.
+Be sure you are working in a Format that includes the Alpha channel, YUVA or RGBA, because you need
+to have transparency for masking to show or will end up with only a black area.
\begin{enumerate}
\item To define a mask, in the Compositor window click on the \textit{Edit mask} icon to get the popup Mask menu. If the menu does not come up, click on the \textit{Show tool info}.
\item On the video, LMB click on the place where you want to start a mask.
In other words, a lot of media does not begin at the \textit{beginning} point and will not be properly rendered until enough data has been read to assemble a picture.
You can increase the thumbnail size, clarity of pixels (memory size) and color mode but it takes a lot more memory.
Change these values in \texttt{Settings $\rightarrow$ Preferences}, Appearance tab, right hand side of the Layout section -- be aware that when you click OK, your session will re-initialize.
-You can also temporarily increase the preview mini-window by use of the mouse wheel up or down.
+You can also temporarily increase the preview mini-window by use of the mouse wheel up or down for media but not for clips.
There are 4 options for the preview mode.