Broken autobeaming when starting with a partial measure with initial grace note

[Daniel Dadap]

Hi all,

When I try to engrave a piece that begins with a partial measure (for a pickup 
beat) and that partial measure begins with a grace note, autobeaming is 
disabled 
for the rest of the piece and can't be re-enabled.

The following example demonstrates the broken autobeaming behavior, including 
on 
the latest development version, 2.13.26-1:

{
\time 4/4
\partial 4 \grace a8 a a
a a a a a a a a
}

A grace note before any other note besides the first note in an initial partial 
measure (including any other note in the initial partial measure) will not 
break 
autobeaming.

Autobeaming also works if, instead of beginning with a partial measure, a piece 
begins with a full measure and a hidden rest; for example:

{
\time 4/4
s2. \grace a8 a a
a a a a a a a a
}

However, this leaves a gap at the beginning of the measure, and will affect 
MIDI 
playback of repeats, among other things.

I'm not sure if this behavior is due to a bug, or due to some incorrect 
preparation of the score on my part. I tried to simplify the test case as much 
as possible, and that's how I discovered the exact conditions required. If it 
is 
a bug, I'm surprised it's gone undiscovered until now, as beginning a piece 
with 
a pickup measure and an initial grace note is quite common, for example, in 
bagpipe music. (I originally discovered this behavior when engraving bagpipe 
music, using the excellent "bagpipe.ly" extension, but found when simplifying 
the test case that "bagpipe.ly" is not a dependency to produce this behavior.)