OK, that seems to be it. It looks like iOS 13 changed the cut-and-paste algorithm slightly and now inserts a space automatically before a pasted URL in a body text box. (I’ll have to check the settings to see if it can be disabled.) But it’s crazy that the preview handles it correctly, but it gets borked when actually posted. (To me it’s bonkers that the forum software renders fundamentally differently in the preview than in the actual post. Why wouldn’t it use the same code path?!?)
One more test: does a spurious space in the url tag matter:
[img width=100]http://...[/img]
works
[img width=100]http://...[/img]
fails
Tooki,
Thank you for this post. I thought it was “just me”

I have been having problems including URL)s in posts for some time. I am using an iPad running iOS 13 and I am finding that when I post a URL, only part of it is ‘captured’ as a URL by the forum (blue text) and the rest of it is ignored and shown as plain black text. Clicking that link then produces a 404 error. Most annoying. I have resorted to using my Windows 7 laptop for posting URL’s now.
Fraser
Thanks BravoV

I am demonstrating how inept I am at posting anything but simple stuff on forums

Fraser
Tooki,
Thank you for this post. I thought it was “just me” 
I have been having problems including URL)s in posts for some time. I am using an iPad running iOS 13 and I am finding that when I post a URL, only part of it is ‘captured’ as a URL by the forum (blue text) and the rest of it is ignored and shown as plain black text. Clicking that link then produces a 404 error. Most annoying. I have resorted to using my Windows 7 laptop for posting URL’s now.
Fraser
This problem is probably as old as the forum itself. And it definitely isn't just you @Fraser.
I am using an iPad running iOS 13...
That sounds very painful. I explored using an iPad for posting on forums for a while but I gave up on it. It is just too inconvenient and frustrating compared to the control you have using a PC screen with keyboard and mouse. A Bluetooth keyboard on the iPad only helped a little.
IMHO iPads are good for reading and browsing, but are not good for content creation.
I am using an iPad running iOS 13...
That sounds very painful. I explored using an iPad for posting on forums for a while but I gave up on it. It is just too inconvenient and frustrating compared to the control you have using a PC screen with keyboard and mouse. A Bluetooth keyboard on the iPad only helped a little.
IMHO iPads are good for reading and browsing, but are not good for content creation.
Exactly.
That can be extended to Android tablets as well.
I have never considered using an external keyboard with a tablet; it would be like putting lipstick on a pig.
I have never considered using an external keyboard with a tablet; it would be like putting lipstick on a pig.
I actually use a bluetooth mechanical keyboard with my iPad frequently. Thanks to a nerve problem and RSI, my hands don't tolerate most keyboards (never mind onscreen typing), but the iPad is often the best tool for many tasks, like messaging. (Facebook messenger on iOS is far superior to using the FB website on a computer.)
I totally agree that an iPad is not an ideal content creation device (though iOS 13 improves this), but in some situations it's quite good. I suspect its content creation strong suit is illustration (where the Apple Pencil manages to far exceed the venerable Wacom tablets in responsiveness). It's certainly not long-form text...
I often end up with my computer, iPad, and iPhone around me when working, as I use each to compliment the others. I like to let each tool do its specialty best.
Totally agree with the challenges of content creation on an iPad. I get very annoyed with mine auto correcting technical words etc to other words and have to keep checking that it is not changing words in my posts (I can turn auto correct off though) The on-screen keyboard is only good for short posts like this really. As for ‘cutting and pasting’ plus image handling, the iPad sucks in my experience. I far prefer to manipulate images, resize them etc on my laptop.
I consider my iPad a window on the internet and a brilliant search tool for finding information. Content creation does not appear to be its forte.
Fraser
I have never considered using an external keyboard with a tablet; it would be like putting lipstick on a pig.
I actually use a bluetooth mechanical keyboard with my iPad frequently. Thanks to a nerve problem and RSI, my hands don't tolerate most keyboards (never mind onscreen typing), but the iPad is often the best tool for many tasks, like messaging. (Facebook messenger on iOS is far superior to using the FB website on a computer.)
...
I like to let each tool do its specialty best.
Very sane, and that's a great example of why personal characteristics are important. I always make that point w.r.t. magnification for PCB operations.
Totally agree with the challenges of content creation on an iPad. I get very annoyed with mine auto correcting technical words etc to other words and have to keep checking that it is not changing words in my posts (I can turn auto correct off though) The on-screen keyboard is only good for short posts like this really. As for ‘cutting and pasting’ plus image handling, the iPad sucks in my experience. I far prefer to manipulate images, resize them etc on my laptop.
FYI, in iOS 13, one can finally shrink them on upload in Safari:
https://www.macrumors.com/how-to/choose-image-size-when-uploading-photo-safari-ios/ At least for adding quick snapshots to forum posts, this is a great feature. I haven't tried it yet, but presumably will at some point. (The forum admins did finally add attachment support for the .jpeg file extension, allowing direct upload from iOS.)
I consider my iPad a window on the internet and a brilliant search tool for finding information. Content creation does not appear to be its forte.
I think that's the wide consensus, with the possible aforementioned exception of illustrations with the stylus.