diff --git a/codetab.ts b/codetab.ts
index 4d18ea5..68bbb18 100644
--- a/codetab.ts
+++ b/codetab.ts
@@ -7,6 +7,7 @@ import { K, ctrlKeyDown, getKeyboardString, keyPressed, shiftKeyDown } from "./k
 import { clipboard, tokenize } from "./deps.ts";
 import { getBuiltins } from "./runcode.ts";
 import { page } from "./viewsheets.ts";
+import { mouseDown, mouseHeld, mousePos } from "./mouse.ts";
 
 const historyDebounceFrames = 20;
 
@@ -433,10 +434,13 @@ const pixelToIndex = (str: string, x: number, y: number) => {
 	const prefix = lines.slice(0, yy).join("\n").length+(yy === 0 ? 0 : 1);
 	const line = lines[yy];
 	let j = 0;
-	while (measureText(line.slice(0, j))+1 < x && j < line.length) {
+	while (measureText(line.slice(0, j)) < x && j < line.length) {
 		j+=1;
 	}
-	return prefix + j;
+	if (measureText(line) < x) {
+		j+=1;
+	}
+	return prefix + Math.max(0, j-1);
 }
 
 const update = async () => {
@@ -451,6 +455,14 @@ const update = async () => {
 		}
 	}
 
+	if (mouseDown()) {
+		const {x, y} = mousePos();
+		state.setSelection(pixelToIndex(state.code, x, y-8));
+	} else if (mouseHeld()) {
+		const {x, y} = mousePos();
+		state.setFocus(pixelToIndex(state.code, x, y-8));
+	}
+
 	const keyboardString = getKeyboardString();
 	if (keyboardString) {
 		state.insertText(keyboardString);
diff --git a/mouse.ts b/mouse.ts
index 77a368d..fa58888 100644
--- a/mouse.ts
+++ b/mouse.ts
@@ -79,6 +79,10 @@ export const refreshMouse = () => {
 	mouseEvents.length = 0;
 }
 
+export const mouseDown = (button: number = M.LEFT) => {
+	return mouseEvents.some(ev => ev.button === button && ev.type === "down");
+}
+
 export const mouseClick = (button: number = M.LEFT) => {
 	return mouseEvents.some(ev => ev.button === button && ev.type === "click");
 }