Thanks martin. Pressing Space did the trick for me.

It's standard Windows rendering of the focus.
You can make it thicker by going to:
Control Panel > Ease of Access > Ease of Access Center > Make the computer easier to see > Make the focus rectangle thicker

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Or you can just press Space in WinSCP to toggle selection of the focused file.

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Or, if you ever deal with multiple files selection, you can use Mark > Restore Selection (Ctrl+Shift+S) to restore the selection before the last upload/operation.

Ah, I can confirm I can still use F5 on the file without needing to find it in the list then selecting it first, which is the way I've been uploading it. My monitor resolution is 4K and after uploading it's almost impossible to see the dotted line around the filename.

I've uploaded 2 images to hopefully show you what I mean.

The selection is lost after an operation. But a focus on a single file remains and the F5 should still work to upload that single file. If you have different experience, please post some screenshot document what is happening (or even better a screencast).

I'm using the latest version of WinSCP. This is probably me being a bit picky but here goes.

When I upload a file, which I'm continually updating, I use F5 then hit ENTER twice to overwrite the remote file.

I'm sure the file I was uploading previously was still highlighted after it's been uploaded so I could easily find it to upload again. Now it loses focus and it isn't easily found.

Is there a setting I can change for this?