Why does sliding edge makes it slide being mirrored?

Question

I have this cube with skewed faces:

I'd like to use Edge Slide tool with Even enabled to move the right edge (selected) to the left without it changing its angle (example). As per manual:

Even E
Forces the edge loop to match the shape of the adjacent edge loop. You can flip to the opposite vertex using F. Use Alt+Wheel to change the control edge.

However what I get is that selected edge becomes mirrored along vertical (in this case Z) axis and that's how it's slided:

Choosing control edge doesn't change that, control edge is selected correctly but it's just mirrored.
In the second part of the gif I enable Flip option of the Edge Slide to choose the unselected face as one the selected will match. It works as expected - the selected and edited face gets orientation of that face. It doesn't for the selected face.

Things like scale+rotation are applied and normals oriented outside.

Why does Edge Slide tool mirror the edge it is sliding?

Answer 1

From my admittedly limited testing I believe it is a side effect of being in face selection mode.

By selecting the whole side face instead of only the desired edges you are unintentionally also edge-sliding the vertical edges perpendicular to the view plane. Somehow the tool's geometry calculation probably gets confused trying to slide both loops at the same time while sharing the same corner vertex.

You can see it behaves as expected when selecting two edges only, but behaves unexpectedly if the side face is selected instead.