xP: fix non-ASCII text completion

xP/public/xP.js

@@ -129,6 +129,9 @@ class RelayRpc extends EventTarget {
 
 // ---- Utilities --------------------------------------------------------------
 
+function utf8Encode(s) { return new TextEncoder().encode(s) }
+function utf8Decode(s) { return new TextDecoder().decode(s) }
+
 // On macOS, the Alt/Option key transforms characters, which basically breaks
 // all event.altKey shortcuts, so require combining them with Control as well
 // on that system.
@@ -598,15 +601,22 @@ let Input = {
 			command: 'BufferComplete',
 			bufferName: bufferCurrent,
 			text: textarea.value,
-			position: textarea.selectionEnd,
+			position: utf8Encode(
+				textarea.value.slice(0, textarea.selectionEnd)).length,
 		}).then(resp => {
 			if (!Input.stamp(textarea).every((v, k) => v === state[k]))
 				return
 
+			let preceding = utf8Encode(textarea.value).slice(0, resp.start)
+			let start = utf8Decode(preceding).length
+
 			// TODO: Somehow display remaining options, or cycle through.
-			if (resp.completions.length)
+			if (resp.completions.length) {
 				textarea.setRangeText(resp.completions[0],
-					resp.start, textarea.selectionEnd, 'end')
+					start, textarea.selectionEnd, 'end')
+			} else {
+				beep()
+			}
 			if (resp.completions.length === 1)
 				textarea.setRangeText(' ',
 					textarea.selectionStart, textarea.selectionEnd, 'end')