In a recent improvement a generalized form element size definition was applied to the textarea field. In the specific definition cols and rows units have been added to the default size units. The size and unit are stored as space delimited pair of values when the values are different from default. Parsing the stored data uses array + operator instead of array_merge when listing the two values. But the + operator overrides numeric keys to the effect that while the list expects two items it only finds one, hence, the undefined offset notice.

The fix is fairly simple and amounts to replacing the + operator with array_merge.

It may be worthwhile noting here that the sizing has no effect on the editor mode of the textarea field and size adjustment in that mode should be done in the template by html and css.