fix(themes/powerline): ensure prompt starts on a new line

test/themes/powerline.base.bats

@@ -8,6 +8,46 @@ function local_setup_file() {
 	load "${BASH_IT?}/themes/powerline/powerline.base.bash"
 }
 
+# Stub a no-op segment so we can run __powerline_prompt_command without
+# sourcing every plugin that the default segments depend on.
+function __powerline_noop_prompt() { :; }
+# _save-and-reload-history is called unconditionally; stub it out.
+function _save-and-reload-history() { :; }
+
+# --- __powerline_prompt_command: missing-newline handler (fixes #2372) ---
+
+@test "powerline base: __powerline_prompt_command overflows the line and returns to column 1" {
+	COLUMNS=20
+	POWERLINE_PROMPT=("noop")
+	run __powerline_prompt_command
+
+	local expected
+	expected="$(printf '%*s\r' "$((COLUMNS - 1))" '')"
+	assert_output --partial "${expected}"
+}
+
+@test "powerline base: __powerline_prompt_command missing-newline sequence is first output" {
+	COLUMNS=20
+	POWERLINE_PROMPT=("noop")
+	run __powerline_prompt_command
+
+	# Confirm the padding+carriage-return appears at the very start, meaning it
+	# is a direct printf rather than anything embedded in PS1 or segments.
+	local prefix
+	prefix="$(printf '%*s\r' "$((COLUMNS - 1))" '')"
+	[[ "${output}" == "${prefix}"* ]]
+}
+
+@test "powerline base: __powerline_prompt_command falls back to 80 columns when COLUMNS is unset" {
+	unset COLUMNS
+	POWERLINE_PROMPT=("noop")
+	run __powerline_prompt_command
+
+	local expected
+	expected="$(printf '%*s\r' 79 '')"
+	assert_output --partial "${expected}"
+}
+
 function local_setup() {
 	LEFT_PROMPT=""
 	SEGMENTS_AT_LEFT=0

themes/powerline/powerline.base.bash

@@ -289,6 +289,11 @@ function __powerline_prompt_command() {
 	local last_status="$?" ## always the first
 	local info prompt_color segment prompt
 
+	# If the previous command left output without a trailing newline, overflow
+	# the rest of the line with spaces so the terminal auto-wraps the cursor
+	# onto a fresh line before the prompt is drawn (mirrors zsh's PROMPT_SP).
+	printf '%*s\r' "$((${COLUMNS:-80} - 1))" ''
+
 	local LEFT_PROMPT=""
 	local SEGMENTS_AT_LEFT=0
 	local LAST_SEGMENT_COLOR=""