SeNioR-

Steve,
go to ACP --> SYSTEM --> Settings --> Posting.
Now go to Links Tab.
Uncheck "Force posted links to open in new windows?"

This was added in IPS 4.4 or 4.5.
Adding backslash means "load from the root namespace for performance reasons".
Using the root namespace allows PHP to use Opcode to run the function which can be much quicker.
More: Strange Whoops! on 4.4.1

It depends on what you want. If you want just to show other members collaborating with the topic author, then a quick plugin will handle it. Bear in mind that the “official” author for the topic will be the real topic creator; no mention to the collaborators in the whole suite. 
If you really want to make a topic have multiple authors, then a change in Core is required. Only that way it will show in user profile that the user is also the creator of a topic, will increase content count, show in search results, etc.

why didn't the upgrade drop the deprecated member_title column after the data was moved to a new custom field? 

I had a different experience and in my opinion it is better to upgrade to 3.4.9 (last version) and then to IPS 4

I think we tried v3 internally with some fairly poor results. I'll mention it to the dev team and get an update.

 

Click 'Source' when posting, add the following to the link's <a> tag:
class="ipsButton ipsButton_primary"  
Don't have a 'Source' button? Search the ACP for 'HTML' and give yourself the ability to post HTML.

Yes 🙂
Here is my custom version 🙂
 

This is not what you have in mind?
 

Maybe this will help?
https://jimiwikman.se/ips/ips-customization/ips-pages/database-templates/category-list/show-cusom-field-with-condition-r3/

Welcome to the Invision Community Nick, glad to have you here and as you can see we like to help if we can so don't be a stranger and of course....enjoy your new Invision Community setup 🙂

Not true. It depends on what the keyword is. That is why long tail strategies exist that makes high rankings far easier. Speed is still just one factor so even if you load in a few milliseconds you will still not rank in the top 10 without offsite strategies. Content is still king and dwell time and quality of content still rank a lot higher than speed. This is why most companies spend money on offsite/onsite SEO or advertising over speed as long as speed is acceptable.
For e-commerce for example they spend more time on CRO than speed and their biggest concern is always duplicate content, conversion funnel and landing pages with targeted offsite strategies. Even though 1 second will give a 15-30% less conversion on mobile in the first funnel that is still less of a concern than the other issues.
Just to be clear: I have no objections against making Invision Community faster or more W3C valid. Everyone wins in that scenario 🙂
I admire you for having the energy to put into such an endeavor and whatever your end goal is I hope you achieve it.

Ignore those instructions just download the zip file (scroll to the bottom of the page), but don't unpack it.  Go to the ACP and head for Customisation/Editor/Toolbars (see below):

 
Once in there click the 'Add Button' button lol

Now you will see that you can upload the zip file you downloaded.

It will only take a couple of seconds to upload.
The button will appear in the tray at the bottom of the screen - just drag it onto the editor toolbar where you want it.  Clicking on the button whilst in the editor toolbars section will allow you to set permissions as to where you want it to be used and who can use it.
You're done - next time you access the editor around the site the button will appear and you can add as many as you want to a page or topic.

 

Just download this:
https://ckeditor.com/cke4/addon/simplebutton
And add it to your editor in the ACP - don't unzip it, just upload as is and add it to your editor:

Gives you this:

Can we now use versions later than 7.6.0 of elasticsearch?
Last one is 7.13.0, but it won't work with IPS 4.5.

Edit: 
I suggested something that is already built in.
Never mind anymore.

Then we know it was a good suggestion 🙂

Edit: 
I suggested something that is already built in.
Never mind anymore.

Edit: 
I suggested something that is already built in.
Never mind anymore.

That is a cache bust key unique to you. Normal browsing would not see that but since you are doing something "odd" it is putting that in to ensure you are not seeing a cached page.

Additionally, you can add to custom.css
.cPost .cPost_contentWrap {overflow: hidden;} .ipsPageHeader h1 {overflow: hidden;}

I would personally be wary to implement this. It is a security feature. I get those sorts of emails all the time from various sites I login to. Yes, they can be redundant but I would rather know the check is happening than not.