Why is my Cylinder not unwrapping square?

Question

I've been trying to study some game assets and replicate them for practice and I cant seem to get the UVs to unwrap the same way. Usually a cylinder unwraps square with a seam down the side, but the top is made up of tris. So 'follow active quads' doesn't work correctly (before triangulating). I've tried unwrapping a few different ways but nothing is giving me the same results as the original asset.

Is blenders UV unwrapping tool even capable of this? Ive extracted the model from a game so im assuming it was unwrapped in an industry software that makes this process simpler.

I've included the blend file below

Original Game Asset

My attempt

Answer 1

You're correct that Follow Active Quads will require quads. In Edit mode select all faces of the model except the top tris and Face > Tris to Quads:

Then in the Last Operation box raise the Max Angles until the base tris convert to quads (I went with 90 degrees):

In the UV Editing Workspace, in the 3D Viewport, after marking a seam (I marked the edge of the existing unwrap) select a face to be the Active Quad, then Select > All (or hotkey A). Then in the UV Editor select the vertices of the Active Quad in pairs and scale each pair to zero on the appropriate axis (SX0 or SY0) until the Active Quad is a rectangle:

In the 3D Viewport, UV > Follow Active Quads (Edit Length Mode: Length Average):

Please note that although this will produce a clean grid of a UV Map for all the quads, the tris at the top will usually need to be scaled and repositioned manually.

Answer 2

I was able to achieve the same result by unwrapping the cylinder and using 'follow active quads' BEFORE merging the top together for the triangulated part. Everything squares up nicely and i've sheared the UV to match the slant. (Im not sure why theres a slant, upon looking at the texture in looks like its meant to spiral around a bit)