Sequence letter aspect ratio not set correctly when aspect ratio is very narrow after middle-mouse drag

Overlapping letters are rendered that are wider than columns in the alignment view if the middle mouse button is clicked and dragged to the left.

Reproduce by importing ensembl example identifier, then show cross-references to create the linked CDS/Protein alignment. click drag with middle mouse to make the columns very narrow - notice the letters in some cases are not correctly scaled. This seems even worse when the 'scale CDNA as Protein' option is enabled, but in general it looks like the fontmetrics calculation is doing something slightly wrong.