I swear I used to understand this stuff.  I plead old age.   Why do I get two different results from the float placements here:

\definefloat[mysidebar][mysidebars]
\setupfloat[mysidebar][leftmargindistance=-.25in,rightmargindistance=-.25in,outer,none]

\defineframedtext [sidebartext]
[width=.4\textwidth,bodyfont=small,corner=round,background=color,backgroundcolor=lightgray,align=flushleft]

\starttext
This is a bit of text
\placefloat[sidebar][leftmargindistance=-.25in,rightmargindistance=-.25in,outer,none]{}{
\startsidebartext
This is a short sidebar
\stopsidebartext
}
And something to follow.

A bit of text
\placemysidebar{}{
\startsidebartext
This is a short sidebar
\stopsidebartext
}
And something to follow.

\stoptext