I have users seeing this problem, particularly on a Samsung Galaxy Tab II, Chrome 42. When entering text into the text wmd-input textarea, the div#wmd-button-bar is above the text input, obscuring the first couple of lines of input.
It appears that due to the width of the viewport, the wmd-button-bar wraps, and this is what causes the obfuscation.
This can be replicated in a desktop browser by resizing the width of the window to below 700px.
I believe this same issue may be related to:
I’m wondering if it makes more sense to check for a min-width, then hide the preview entirely (it seems to work this way on iOS as far as I can tell - no preview, which is nice.