It is infuriatingly terrible. A regression. So many times I've had similar woes with the code block and quote block mechanisms too.
Another truly bizarre feature I noted and sent a bug report about was that when adding an image to an "Action" the _minimum_ size requirement is 512px by 512px. For an image that is never rendered larger than 64x64.
All code sucks now, not just blocks. Inline code screws up at least half the time I use it; it tries to guess what I want, and sometimes doesn't end when I close it with another backtick. And, when going back and editing it sometimes carries the formatting outside of where I put the backticks.
So frustrating. I hate having to think every time I insert code and re-do it about half the time; I'd rather have no formatting (just show me the markdown as-is) than this mess.
The worst thing about it is how fucking easy it would be to fix. Just treat the backticks as characters when backspacing or navigating with the arrows, that way you can 'enter' or 'exit' a code block reliably, and all previous muscle memory from the markdown editor would carry over.
But no, apparently that's too hard for a company that's worth more dollars than the number of seconds any of us have been alive.
I think there's a battle between nerds and non-nerds, and maybe in between is the worst place to land. I know some of my projects' users complain a lot about the lack of a WYSIWYG editor in our forums and issue tracker (we use Markdown, displayed plain until previewed or saved). Some people hate it. But I would hate a standard WYSIWYG editor, so we just accept the hate. Every Markdown-ish WYSIWYG editor I've tried has sucked...so I don't know that it actually is easy to get right (if it is even possible, where is the good example?).
Slack serves a mix of nerds and non-nerds, with non-nerds becoming a bigger and bigger portion of their user base over time. I can only assume it will become less and less enjoyable for nerds in service to the goal of serving their growing non-nerd users. For my own projects, I don't foresee myself ever using Slack (I use it for work). It feels like a decent product getting worse with time as it tries to be all things to all people.
The changing behaviour of enter/shift+enter in code block drives me absolutely crazy.
Normally, shift+enter is newline, enter is send. However, inside a code block, it's the opposite. I constantly forget this and press shift+enter for a newline while in the code block and accidentally send a half-finished message.
Sad to see the new WYSIWYG editor does exactly the same thing.
I definitely don't remember ever seeing or changing that setting, but it is 1000 times better with it off. Guess it's worth looking through an app's settings every once in a while, no matter how long you've been using it, just to see if there's anything new (or maybe forgotten) that would improve things for you.
On a high-density display, that 64px x 64px image covers a lot more surface area than 64x64 actual pixels on the physical display. I suspect that this 512x512px requirement is related to scaling factors on devices with high DPI displays.
I'm not even sure why we refer to logical pixels (a.k.a. CSS pixels, etc.) anymore, for exactly the reason you mentioned. We should be saying things like "this only needs to be 512px x 512px because it's never rendered larger than two degrees of viewing angle" or something to that effect. But I guess that's hard to think about... ems could be a good compromise.
There's probably a degree of forward-compatibility intended with that requirement. Slack may only display the image at 64x64 (128x128 at 2x dpi) right now, but they want to avoid requiring you to upload a new image if, at some point in the future, they implement a UI with larger action images.
That's likely due to HiDPI displays. A regular display is probably in the 100 DPI range, but we have phones that are in the 400 DPI range, so that icon size suddenly isn't so big anymore.
Another truly bizarre feature I noted and sent a bug report about was that when adding an image to an "Action" the _minimum_ size requirement is 512px by 512px. For an image that is never rendered larger than 64x64.