Action Shortcut
Navigate to Chat Area (Messages) Press Tab repeatedly until the chat area is focused.
Navigate Messages in Chat Up Arrow / Down Arrow
Navigate the Left Sidebar (Servers/Channels) Alt + Up Arrow / Alt + Down Arrow
Reply to a Message Shift + R
React to a Message Shift + \
Edit Your Last Message Up Arrow (in text box)
Jump to Newest Message Ctrl + Down Arrow
Mark Channel as Read Esc
Search in Chat Ctrl + F
Open Quick Emoji Picker Ctrl + E

Other useful shortcuts:

Action Shortcut
Switch Between Servers Ctrl + Alt + Up/Down Arrow
Switch Between Channels Alt + Up/Down Arrow
Open Emoji Picker Ctrl + E
Start a Call Ctrl + '
Toggle Mute Ctrl + Shift + M
Toggle Deafen Ctrl + Shift + D
Open User Settings Ctrl + ,
Toggle Overlay Shift +
Pin Message in Chat Highlight message, Enter, and select Pin
React to Message Highlight message, Enter, and select Add Reaction
Mark Channel as Read Esc
Jump to Newest Message Ctrl + Down Arrow
Search in Discord Ctrl + F
Quick Edit Last Message Arrow Up (in the text box)
Delete Message Highlight message, Enter, and select Delete
Navigate Mentions/Replies Ctrl + I
Quick Switcher Ctrl + K