Are projected lines working correctly in this sketch?

[SonicFlow]

I shelled a body, and I'm trying to clean up and thicken some areas inside.  I want sketch lines that coincident to the edges inside, so they became projected, which should be fine.  However, when I attach a regular sketch line to it (with Coincident), I lose the ability to move the line or dimension it. 

If I draw a line and dimension it, and then make it coincident to one of the purple lines, the dimension disappears. 

I got my triangle sketch to function (for an extrude), by instead using the .12 dimension from the outside, but I shouldn't need that.  

By they way in view settings is there a way to turn off visibility of projected lines? 

[davebYYPCU]

Dimensions that disappear - means you are in creating a 3d sketch with the operation that causes the dimension to vanish.

Make the sketch with the projected lines in it hidden with the Eyeball icon.

Might help...

[SonicFlow]

Thanks davbYYPCU!  That was correct, it was jumping the end point of the triangle out of plane, and directly coincident with the edge. I thought that purple line was projected on to the sketch plane, but it seems to be just an edge that is turned purple.

So how do I constrain my sketch lines to the sketch plane? - while making use of an edge that is a few mm behind the sketch plane? I turned off two options about auto project.  Still does it.

It seems like it should be very easy (default behavior) to avoid making a 3D sketch. 

[davebYYPCU · Accepted solution]

Untick 3d sketch in the Sketch Palette, - I think you are saying the Projected lines were correct, but new geometry went off plane.  That section / Slice makes the pic harder to understand.

I project what I need first, then hide the source, helps prevent that sort of snapping.

Might help....