Disable autosizing when textarea is manually resized

There's no great way to currently test for this with js, so fall back to
using mouse events and detecting a change when the mouse is down.

src/autosize.js

@@ -61,6 +61,32 @@ function assign(ta) {
 			heightOffset = 0;
 		}
 
+		var mouseDownListener = () => {
+			// suspend maximum height so that user can resize to greater
+			// than it
+			var oldMaxHeight = ta.style['max-height'];
+			ta.style.height = ta.offsetHeight + 'px';
+			ta.style['max-height'] = null;
+
+			var heightBefore = ta.clientHeight;
+			var mouseMoveListener = () => {
+				var heightAfter = ta.clientHeight;
+				if (heightAfter !== heightBefore) {
+					oldMaxHeight = null;
+					ta.removeEventListener('mousemove', mouseMoveListener);
+					ta.removeEventListener('mousedown', mouseDownListener);
+					destroy(ta);
+				}
+			};
+			ta.addEventListener('mousemove', mouseMoveListener);
+			ta.addEventListener('mouseup', () => {
+				ta.style['max-height'] = oldMaxHeight;
+				ta.removeEventListener('mouseup', this);
+				ta.removeEventListener('mousemove', mouseMoveListener);
+			});
+		};
+		ta.addEventListener('mousedown', mouseDownListener);
+
 		update();
 	}