I guess you've manually edited your text/hex viewer config to make it open files larger than 10MB?
I don't think it is Opus itself locking the file there. It is most likely your anti-virus, still inspecting it (which some virus scanners spend a long time on with some installers), since the Opus hex viewer does not keep files open. Or you've still got the installer running in the background, perhaps. (Note that some installers stay running for a while after they appear to have exited. Opus's own one does that, in fact. Seems to be something some InstallShield installers do, presumably waiting to clean up some temp-files or something.)
If I select the Thunderbird installer you suggested, the viewer doesn't display it at all as it is too large (and I can't configure the viewer to show a file larger than that as 9999 KB is the max the UI allows me to put in).
If I try with a smaller EXE, the viewer shows it as hex; I click shift-delete, the file is deleted.
So I don't think the problem is the viewer itself locking the file, unless the text/hex viewer only locks large files, but I can't configure it to open such a large file in the first place.
Same problem here. It's not possible to SHIFT-DEL the file if it was recently downloaded and the content is shown in the viewer pane.
Initially I recognized that problem, when moving files around. It is the same bug.
Instead of SHIFT-DEL the file, please try to move the file to an other partition. In this case the file copy dialog appears, than the UAC comes up (I accept) and finally an error dialog is shown saying that the file can't be moved since it is locked by a process. And in the process list only DOpus itself is listed.
This bug only occurs if the content of a recently downloaded file is shown in the viewer pane. Maybe only exe files are affected.
The text viewer keeps the file open if the file is larger than 2MB (the file I was trying with was smaller, and I didn't realise the size limit was optional or off by default, but you're right about that), and it wasn't being closed at the right time when the delete happened.
Sorry for my incorrect theories above. I didn't realise the text viewer did this with larger files.
Video files may be locked open by the video player, and we're not always able to close them in a timely manner.
The viewer tries to release files when needed, and many of our own viewers try to avoid keeping files locked in the first place, but it is not always possible with every filetype, especially when 3rd party code is involved. It's never going to be perfect. Just close the viewer if it's causing you problems, especially with video files which involve codecs that do all kinds of crazy things.