Alignment is misaligned in wrapped mode with annotation and exceptions thrown when only a few columns shown in wrapped mode

Noticed while testing JAL-3364.
Load uniref50.fa, show annotations, wrapped mode, make panel narrow, export image as PNG.
Sequence and annotation labels creep out of alignment with rows, by about 2 pixels per block. See attached example.
Without annotation shown, it is correct.
Also wrong on screen (restrict to a small number of sequences, one annotation, narrow panel, to see the effect clearly.)