Support <Shift><Enter> through multiple nested tmux's and alacritty

config/alacritty/alacritty.toml

@@ -22,3 +22,16 @@ style = "Regular"
 import = [
     "~/.config/alacritty/themes/themes/gruvbox_dark.toml"
 ]
+
+[keyboard]
+# Alacritty doesn't implement the CSI-u/Kitty keyboard protocol that lets a
+# terminal report Shift+Enter as distinct from plain Enter, so it needs an
+# explicit binding. Sending a raw line feed makes Shift+Enter equivalent to
+# Ctrl+J, which Claude Code (and most other terminal apps) already treats
+# as "insert newline" with zero further configuration -- this works
+# unchanged through any depth of nested tmux, since it's just a literal
+# byte rather than an escape sequence that needs to be interpreted along
+# the way.
+bindings = [
+    { key = "Return", mods = "Shift", chars = "\n" },
+]

config/tmux.conf

@@ -58,6 +58,15 @@ bind C-y run 'tmux save-buffer - | reattach-to-user-namespace pbcopy'
 # option, so only set it when the running tmux is new enough.
 if-shell "tmux -V | awk '{print $2}' | awk -F. '{if ($1*1000+$2 >= 3003) exit 0; else exit 1}'" 'set -g allow-passthrough on'
 
+# Let tmux tell Shift+Enter apart from plain Enter and forward that
+# distinction inward, so Claude Code's Shift+Enter-inserts-newline shortcut
+# works. extended-keys/terminal-features need tmux >= 3.2. The terminal
+# name is matched with '*' rather than the commonly-suggested 'xterm*',
+# because a nested tmux's "terminal" is the outer tmux, which reports
+# itself using default-terminal above (screen-256color) -- 'xterm*' would
+# never match that, breaking the nested case this is meant to support.
+if-shell "tmux -V | awk '{print $2}' | awk -F. '{if ($1*1000+$2 >= 3002) exit 0; else exit 1}'" 'set -s extended-keys on; set -as terminal-features "*:extkeys"'
+
 # Also make tmux's own copies (mouse-drag select in copy-mode, prefix-[ y,
 # etc.) reach the real system CLIPBOARD via OSC 52 -- not just PRIMARY/
 # middle-click -- by declaring the "Ms" (set-clipboard) terminfo capability