Lubuntu 18.04, PCManFM 1.2.5, PDF Thumbnail Previews

Hello forum,

using Lubuntu 18.04 with PCManFM 1.2.5 here.

Since a few weeks PDF Thumbnail Previews of newly downloaded PDF-documents aren’t shown anymore. Thumbnail Previews of formerly downloaded PDF-documents are still shown.

I wonder if this is due to some update or some other issue. And how I can fix it.

Anyone experiencing the same?
Anyone having some idea how to solve this issue?

The files’ sizes do not exceed the 32768 KB limit (when thumnail previews should not be generated/shown by design/configuration).

Thanks in advance!


This is really strange. I have to I agree. A fresh installation of 18.04.1 does not have the problem, but one that is updated does. Updating evince is all it takes to make it not work.

Oddly, this doesn’t seem to make any significant difference (it only adds to the supported MIME types) to the /usr/share/thumbnailers/evince.thumbnailer file. In fact, copying the original content back over doesn’t make for any difference.

Additionally, one should be able to evince-thumbnailer /path/to/pdf /path/to/png and end up with a PNG thumbnail. This works fine in the live 18.04.1, but in the updated installed system, it fails silently with an odd exit code of 254 which I can’t find referenced in the code but I think the negative return values might actually be what that refers to. Still doesn’t help that much. There’s also supposed to be ~/.cache/thumbnails which includes a fail folder if there are issues. Neither it nor its parent are there.

There are plenty of bugs related to thumbnails but most of them seem to be crashes and not particularly relevant, either. And there doesn’t seem to be anything good upstream, either. Weird.

Looking at the changelog there appears to be a security bug that was fixed, making the thumbnailer more restricted. I think this is behind a different but related issue in Cosmic. The workaround there doesn’t help. It makes things worse, actually. Still, it smells like the problem would be around there somewhere.

Curiously, using the live Ubuntu system and upgrading evince causes a fail, too, though with a different error. Exit code 255 and Error opening directory "/usr/lib/x86_64-linux-gnu/evince/4/backends": Permission denied, which still reeks of that aforementioned issue, but the workaround again doesn’t help.

I need to investigate some more to figure out what is amiss.

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I’ve been adding what I can figure out to my previous reply, but I’ll mention something else: all works well in LXQt. Upgrade to 19.04. You’ll end up with a more developed, supported, and modern set up to boot.

Hi wxl,

many thanks for your quick replies and investigation. I’m glad, you could reproduce this issue.

I had chosen Lubuntu 18.04 because of its long term support and low resource consumption, so I would be happy to not have to upgrade to 19.04 to get the PDF thumnails back.

Looking forward to further findings. Many thanks again!

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What I can say as I don’t think this is something that one can just toggle a normal setting to fix nor is it something that is specific to Lubuntu. Ultimately, it’s likely to result in a bug report. Feel free to start that process and share the results here.

Regarding 18.04, it doesn’t have support for much longer. Once its support has ended, there’s no LXDE version of Lubuntu to upgrade to. Additionally, many of the upstreams (LXDE included) are not very supportive. So when we say 18.04 is a “long term support” we mean that within this context, which is to say, it’s not that well supported.

Finally, the resource consumption on the two is near identical. Would LXDE finally resolve their long-standing issue of using GTK3 instead of the old GTK2 (which I know is responsible for causing pcmanfm to crash for some folks), it would be worse than LXQt.

LXDE/18.04 is really a dead end.

Thanks for your hints. I am not familiar with the bug report process. Do you think it would result in a fix within an reasonable time? (say < 1 month)

Regarding 18.04 as far as I am informed its supports ends by April 2021 (opposed to December 2019 for 19.04).

Sorry for off-topic here, just to give some information about my situation:.
Before moving to LXQt I would want to find out if with LXQt Lubuntu is still the best choice for me (compared to e.g. XUbuntu) - having heard an opinion saying it isn’t. For my use case I prefer a solution which is as lean as possible (mainly talking about CPU and RAM, but also storage consumption) and needs as little maintenance effort as possible while security updates are considered mandatory. (so LTS versions preferred and upgrades every year not really appreciated)
Actually I unfortunately lack time and storage to do testing with different distributions.

Thanks again for your amazingly responsive support.

Yes 19.04 has a short life span (being a regular 9 month release), but it’s upgrade path is to 19.10, then 20.04 LTS, and forward.

As it stands now Lubuntu 18.04 may require a re-install to go to 20.04 LTS due to the change from LXDE to LXQt (or possibly be more problematic if not re-installed making a re-install the likely safer option). This is part of why 19.04 can be seen as having the better support moving forward. Note: I’m no wxl and giving a user opinion here.

As for comparison with say Xubuntu. Your use case will influence which will be more efficient for yourself, as LXQt (Lubuntu 18.10 & up) use the Qt libraries, so if you use programs that use Qt it’ll perform better [generally] than XFCE/Xubuntu which relies on GTK+ libs. However the reverse will also be true (ie. Xubuntu will be better when using GTK+ programs generally). The reason for this is you don’t have to waste memory by having two sets of libraries in memory at the same time, one set needed for your desktop, another set for your applications. The XFCE GTK+2 to GTK+3 conversion is ~99% complete done (it’s in pre-release2 position right now). The LXDE desktop using GTK+2 libs, but most GTK+ programs wanting GTK+3 libs can waste memory too. My point here is more than just the desktop; your choice of programs has a huge influence on which desktop is best for you.

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Hi Chris,

thanks for your hint.

99% of my Lubuntu use currently is Webbrowsing (with Mozilla Firefox) plus intermittent PDF-viewing. I had left original Ubuntu as after some update youtube videos became stagnant which was healed by changing to Lubuntu.

So what might be the best choice for me?

There is no problem with Lubuntu; but try and pick Qt based apps if you can. Qt is also used by KDE, and also found in Android & Windows apps. GTK+ (or more recently GTK) is used by GNOME & variants only (XFCE, MATE, Budgie etc).

I’ll provide example that may help, Lubuntu 18.10 up comes with featherpad, looking up it’s requirements shows or packages that are required on your system, and likely will be required in memory when the app is running (or when some functions are used).

In comparison look at mousepad, the text editor that comes with Xubuntu and you’ll note different libs ( My point is not the number of dependencies (two small ones can use less memory than a large one and Qt/KDE is very modular), but if you want a very lean machine be careful with choosing your apps (esp. if you have less than 4GB of ram).

I tested Lubuntu & Xubuntu 18.10 & 19.04 using x86 hardware with single-core cpu and only 1-1.5gb of ram and was impressed (until x86 ISOs stopped being produced in Dec-2018). I found I preferred the experience on some hardware better than others (but we can’t often easily change our hardware, eg. I preferred laptops with intel graphics over originally originally more expensive radeon graphics), and with rare exception I preferred Lubuntu.

Thankfully I don’t have to ‘live’ on the 1gb hardware as yes the tasks took longer (primarily closing one app when I wanted to switch to another to avoid paging due to the limited ram), and I could do everything I do now on my main box (with 8gb ram), I just had to be more careful with installing programs, and what programs ran at any one time.

Browsing can be annoying, as most sites let your machine (or browser) do all the work generating the page and deciding what you’ll see. As a result I’d say here avoid adding extensions if you can (they use memory), and have as few tabs as possible open at a time (I am very careful when using 2gb of ram or less). There are lighter browsers (eg. falkon) however experiences with it can vary on your hardware & sites you visit.

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On your machine, on the command line, simply issue the command ubuntu-bug evince and follow the prompts. Make sure in the bug description to explain the issue is with the thumbnailer that comes with evince and include the aforementioned information.

If this was an LXDE component, I’d say no way. As a GNOME component, and one that does not work in Ubuntu proper, I think it’s possible. There’s no guarantee, though. Most of Ubuntu exists because of the work of volunteers.

What was this opinion based on?

The “L” in LXQt means the same thing as the one in LXDE: “lightweight.”

I understand that you mean you want to do few upgrades, but I think another thing to consider is that LXDE is more likely to have problems and less likely to see resolutions to those problems. Also the other issue with LTS is that it’s old software. Old software is less likely to have new features that introduce new bugs but it’s also more likely to have old bugs. That said, as someone who is responsible for maintaining a workplace running Linux, I have found that LTS does not necessarily mean less maintenance, especially given how painless the upgrades are.

Then trust me, you want Lubuntu LXQt!

Also I think @guiverc is pretty spot on, except that I’ll mention two things:

  1. Qt and GTK can live happily together on the same system. Admittedly, there will be some additional memory usage, which is why we have worked so hard to provide LXQt to use Qt apps as much as possible. In many cases, though, this would be very minimal.
  2. The one application that is responsible for most of your memory usage and the one area where LXQt Lubuntu fails to use Qt is the web browser. I would urge you to consider using Falkon instead. We were going to include it as standard in 18.10 but it was a little crashy at that time. Since then, I know we have at least one team member that uses it regularly.

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