I just updated to the latest Factorio Beta - 0.18.34. The button in the top left of my screen which usually showed my tape measure no longer works. It lacks an icon, it's the wrong size and position (too wide, not high enough, et too high and too far to the right) and clicking it does nothing. It worked fine in 0.17, but no longer does in the new version.

Are you using the latest mod version as pretty sure I removed the icon in top left when I added the shortcut bar button.

I'm using the latest one the in-game Mods menu would download, but I went to check anyway. I'm using 18.1.5. However, when I went in-game to check for the issue, it wasn't there. So either this was caused by another mod or else I WAS using an old version which I later automatically updated.

Just to be clear - there is NOT supposed to be a UI button to pull out the tape measure any more? If so, then my apologies - please disregard.

Correct, in the latest version there should be no icon in the top left to get a tape measure.
Yea sounds like an old version or the old GUI was left behind on mod update and has since been cleared out by some other action done.