I want to make some lights with a retractable beam: the beam is minimized and centered into the root prim when inactive. The light can rotate while the beam is active or not, and I want only one script in the root prim to control the beam.

FIXED: do not apply any rotation! In fact, when setting PRIM_POS_LOCAL, it seems OpenSim also computes the offset due to root rotation.

Here is a picture of the action:

The active size of the beam is saved before it's hidden, so it can be used when restoring it. I tried several methods without success. For me, this one was the right one:

integer BEAM_LINK = 2
// returns the new local position of the beam from its stored size
vector restoreBeam(vector beamSize) {
  vector rootSize = llGetScale();
  rotation rootRot = llGetRot();

  // calculate the offset from a "zero-rotation" root prim
  vector beamOffset = <0.0, 0.0, rootSize.z / 2 + beamSize.z / 2>;
  // the wanted beam position relative to the root
  // this is simplified considering the beam has a zero rotation relative to the root
  vector beamPos = beamOffset * rootRot;
}

All is ok as long are there is no rotation, but as soon as there is one, here is the result:

I tried several other formulas, like converting first to region coordinates, applying the rotation, then removing the root position from the result.