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

test/themes/powerline.base.bats

@@ -0,0 +1,32 @@
+# shellcheck shell=bats
+# shellcheck disable=SC2034 # Variables consumed by externally-loaded powerline functions.
+
+load "${MAIN_BASH_IT_DIR?}/test/test_helper.bash"
+
+function local_setup_file() {
+	setup_libs "colors"
+	load "${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 outputs missing-newline escape sequence to stdout" {
+	POWERLINE_PROMPT=("noop")
+	run __powerline_prompt_command
+	assert_output --partial $'\e[7m%\e[0m\r\e[K'
+}
+
+@test "powerline base: __powerline_prompt_command missing-newline sequence is first output" {
+	POWERLINE_PROMPT=("noop")
+	run __powerline_prompt_command
+	# Use bash pattern match to confirm the sequence appears at the very start,
+	# meaning it is a direct printf rather than anything embedded in PS1 or segments.
+	local prefix=$'\e[7m%\e[0m\r\e[K'
+	[[ "${output}" == "${prefix}"* ]]
+}

themes/powerline/powerline.base.bash

@@ -290,6 +290,10 @@ function __powerline_prompt_command() {
 	local beginning_of_line='\[\e[G\]'
 	local info prompt_color segment prompt
 
+	# Reversed % marks where a missing trailing newline was; \r\e[K then
+	# clears the line so the prompt always starts cleanly at column 1.
+	printf '%b' '\e[7m%\e[0m\r\e[K'
+
 	local LEFT_PROMPT=""
 	local SEGMENTS_AT_LEFT=0
 	local LAST_SEGMENT_COLOR=""