I have a tablix with the middle column text hidden until the left column is expanded. All the text fits, but when I click the expand "+" the whole tablix width expands, causing other items on the report to move with it. Strangely, it never happens for the 1st row, but for any other row the problem occurs. There's no reason for it to widen, as there's no text which is longer than the fields. I've tried with 2 different browsers and I get the same problem. However, it doesn't occur in the "Preview" view in BIDS/Data Tools.
Edit:
Also, even more strangely expanding the first row seems to set the width back to normal, regardless of what other rows are expanded.