UV image editor Island selection mode fails in some condition
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Description

Hi! There are 2 basic cubes in attached .blend file (on layer 1 and 2). Both cubes unwrapped using simple scheme - 2 faces packed together (seam around pair of faces). As result of unwrap we should have 3 separate islands (6/2=3 if faces are adjacent). In UV image editor select island select mode and try to select each island for cube on layer 1 and for cube on layer 2. In first case 2 islands merge in one 4 face island (wrong behaviour) but in second we have three 2 faces island - that should be right behaviour I think. The behaviour defers because of order we select edges to make seam (and its weird).
I think it has some roots in vertex order for islands and their location but it works the same (wrong scenario) in all shared location selection modes.

Details

Type
Bug

I can at least confirm this behaviour with the attached blend file. Although I could not recreate it with the default cube or from another added cube. Is there a way to recreate this behaviour from the default cube? That may help in finding the real problem here.

32-bit Kubuntu 12.04
Intel Core2 Quad Q6600@ 2.40GHz
NVidia 8800 GT - Driver version 304.43
Blender 2.64 r51483

I noticed if you add even a small amount of margin between the islands that they separate and no longer are selected together. You can add margin by first doing the unwrap from 3d view, and then in the operator properties dragging the margin setting.

Hi Anthony! You can recreate it by choosing different order in which you select edges to make a seam. Internals of this behaviour I don`t understand quite right.
There are some "workarounds" how to separate islands but in some cases they are not acceptable. For example we want place one UV-island on top of another similar island. After they "connected" we should use "workaround" again. Recently I wrote plugin for Blender which copies one uv-island in uv-coords of another (if they have same structure). Demo video is here http://www.youtube.com/watch?v=cNwvc6F-8Ik . After you watch video you can understand why this bug will interfere the smooth work-flow.

I think brecht added some small margin to the default unwrap operator a few days ago. Can you check with a new build?

Indeed, you should unwrap with a margin. The seams don't define connectivity in the UV editor, it's all based on the UV coordinates, and if they happen to be in the same place then they are considered connected. I can understand there's cases where this is not so useful but there's also cases where you unwrap and don't use seams .. so neither solution is perfect. I don't consider the current behavior a bug.

I did notice an issue where the default startup.blend had margin 0.0 still, fixed that now.