Cap length at clientHeight for new requests

lib/snapshot.js

@@ -185,11 +185,36 @@ function take_snapshot( p_uri, p_filename, p_width, p_height, screen_width, scre
 			await page.waitFor( 2000 );
 			makeDirIfRequired( _path.dirname( p_filename ) );
 
+			// Backwards compatibility requires we continue to allow
+			// screenshots longer than the target.
+			//
+			// However, as we support longer pages guessing the wrong
+			// length becomes uglier and more wasteful, so we'd like
+			// to limit the clip length to the screen size.
+			//
+			// Prior to an actual `/mshots/v2/` bump we can compromise by
+			// limiting to the clientHeight only on requests that have
+			// specified the new screen_width/screen_height parameter.
+			let clientWidth = Number.MAX_SAFE_INTEGER;
+			let clientHeight = Number.MAX_SAFE_INTEGER;
+			if ( screen_width !== p_width || screen_height !== p_height ) {
+				[ clientWidth, clientHeight ] = await page.evaluate(
+					() => [ document.body.clientWidth, document.body.clientHeight ] );
+			}
+
+			const snapshotHeight = screen_height
+				? Math.min( screen_height, clientHeight )
+				: p_height;
+
+			const snapshotWidth = screen_width
+				? Math.min( screen_width, clientWidth )
+				: p_width;
+
 			await page.screenshot( { path: p_filename, fullPage: false, type: 'jpeg', quality: 90, clip: {
 				x: 0,
 				y: 0,
-				width: screen_width || p_width,
-				height: screen_height || p_height
+				width: snapshotWidth,
+				height: snapshotHeight
 			} } );
 
 			await page.close();