[Vm-dev] primitiveDisplayString fails on empty Strings

[Fabio Niephaus]

Hi all,

I noticed that BitBlt's primitiveDisplayString fails relatively often and
then found out that it's sometimes called to draw empty strings.

`(ByteString allInstances select: [:ea | ea isEmpty]) size` revealed that
there are 2245 empty strings in my image and I assume some of them must be
either in the FrameRateMorph morph or in the Clock morph.

Anyway...looking at BitBlt>>#primDisplayString:from:to:map:xTable:kern:,
the fallback code basically does nothing if stopIndex is zero (`startIndex
to: stopIndex do: ...`) which makes sense.

What I think doesn't make sense is that the primitive fails in the first
place. Instead of failing if stopIndex is not greater 0, I would suggest to
use: `stopIndex < 1 ifTrue: [^ interpreterProxy pop: 6].` which cleans up
the stack, leaves the receiver, and returns early.

Let me know what you think!


Fabio
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