The new-look interface for writing solutions uses half a column to give a box with "do"s and "don't"s when writing solutions, which is fine. However, this only leaves an edit box half a column wide in which to type the solution, and this half-width persists when previewing the solution, but not when the solution is finally posted.
As Calvin's guide points out, an important rule of writing solutions is that your formulae, particularly your display formulae, are easily readable on the page. The way that formulae (arrays of formulae in particular) are best typeset is very dependent on the linewidth available. That the new method of posting a solution no longer offers a preview screen of the same size as the final output is not good. It is much harder to proof-read when continually scrolling through narrow columns, and the need for adjustments to formulae, to make them fit the display output neatly, will no longer be evident when writing solutions.
Please can we have the full-width preview option back? I note that this box of tips appears beneath the edit box in the mobile version, which seems a better place for it.