Create a horizontal arrangement
Inside the horizontal arrangement place three vertical arrangements
inside each vertical arrangement place a textbox.
Now, only using blocks (no extensions), make it so that all three textboxes maintain the same height as each other, regardless of the content / what is typed into in each textbox.
Show your blocks/settings here in this topic. Have fun 
That will most certainly work on initialise, but what happens if you change the number of rows in a textbox, e.g. the length of text typed in to the textbox?
It is close
I get this if none of the textboxes have focus after a "shorter" textbox is selected
Well it seems no-one else is up for the challenge, herewith is my solution for a single row of textboxes:
The solution is really all about firstly setting the textboxes to automatic, to get them all displaying all their content, then getting their heights, then setting all the textboxes to the greatest height, using lost focus to complete the routine. A touch/press/click is required somewhere to make it happen (I believe using a clock would be too intrusive for the user, but may be possible). I used @ABG's max procedure to get the greatest height.
A clock is needed to give the textboxes enough time to get their heights, a setting of 50ms works.
The routine is completed when the edited textbox loses focus, i.e. when another textbox is touched
TextBoxRow.aia (3.6 KB)
I also prepared a work in progress soloution for handling multiple rows of textboxes in the same layout. The workflow didn't want to let me just set heights for a single row, so heights for all rows are set after a change in a textbox. Not completely figured this one out yet....
A clock is needed to give the textboxes enough time to get their heights, a setting of 50ms works, but this may need to be longer for many rows.
The routine is completed when the edited textbox loses focus, i.e. when another textbox is touched
TextBoxRow2.aia (4.7 KB)
A bit more development required for real world use, to make this truly dynamic for any number of rows...
Needs to be more dynamic to handle the size of the content in any of the textboxes 
In the last few days I have seen that (my) gif animations are no longer displayed directly.
These must first be clicked to start. 
Yes, appears to be a new feature in Discourse. For some gifs this is an improvement, but now we will have to write "click on the image" to activate gif / animation in the post as well 
Finally worked out the solution for multiple rows. The result can be found in my Dynamic Table demo
I'll look and see if there's an option to change this. We had to do the upgrade because of a remote code execution exploit in the version of Discourse we were on, which means you get the bad with the good.
After reviewing the settings and some related PRs, I haven't been able to identify a way of turning this "feature" off.
Seems to still work in some cases
