I hope this level of nitpicking isn't out of order :)

The tab key doesn't quite work as expected in parts of the UI.

Share Message dialog/Start thread dialog: Pressing tab does nothing when the text field is focused, while pressing shift+tab inserts a tab character. When a different element (button or checkbox) is focused, tab navigation works until the text field gets focus again.

Emoji Reaction dialog: Tab works for cycling between elements (though arguably the emoji size selector button should be the second tab stop rather than the third - this button also lacks a tooltip), but there is no way to select an emoji with the keyboard - I can only scroll the list up and down with the arrow keys. It would be nice if the arrow keys instead moved through the image list.

Main window: Pressing tab does nothing while shift+tab inserts a tab character. Perhaps tab/shift+tab could navigate between the main text field, the sidebar, the tabs list, the buttons in the toolbar and the message area. If the latter is focused the arrow keys could be used to select messages (like the current left-click behaviour). When a message is selected shift+F10 or the menu key would trigger the context menu for the selected message.

Tab is used for tab completion. It cannot be overloaded to do other things in the main chat window. Trying to make it do other stuff in special cases, like if there's no text, will just make it highly prone to error and jankiness and unexpected behavior by users, including myself.

The share message and start thread dialogs have a chat text style input, where tab completion is important. Tab key also cannot be overloaded here. It's true that there needs to be some other way to confirm the action in these dialogs without requiring the mouse. But not by tab cycling (unless a screen reader is attached, in which case the UI is operating in a slightly different tab focusing mode.)

Actually, please just break this up into different feature requests. I don't know how to handle a vague ticket like this with a bunch of different stuff mixed together. I'm not sure which are feature requests or thinking out loud. Or if you want to put multiple related stuff into one ticket, please list the things more clearly. I can't easily read this ticket and figure out which is the current behavior and which is the behavior that you want.

Or think about it like this: what am I supposed to do with a ticket like this? There is no way for me to ever mark it as completed, because there is no way to complete it.