Use keypress events for arrow zooming.

The plus and minus key codes are not consistent across browsers. The char codes
appear to be more consistent, so we use those; note that this also has the
side-effect of enabling repeated zooming if the plus or minus key is held down.
This makes zooming more consistent with the current (custom repeating) pan
behavior.

polymaps.js

@@ -1351,7 +1351,7 @@ po.wheel = function() {
 var bug40441 = /WebKit\/533/.test(navigator.userAgent) ? -1 : 0;
 po.arrow = function() {
   var arrow = {},
-      key = {left: 0, right: 0, up: 0, down: 0, plus: 0, minus: 0},
+      key = {left: 0, right: 0, up: 0, down: 0},
       last = 0,
       repeatTimer,
       repeatDelay = 250,
@@ -1362,7 +1362,7 @@ po.arrow = function() {
 
   function keydown(e) {
     if (e.ctrlKey || e.altKey || e.metaKey) return;
-    var now = Date.now(), dx = 0, dy = 0, dz = 0;
+    var now = Date.now(), dx = 0, dy = 0;
     switch (e.keyCode) {
       case 37: {
         if (!key.left) {
@@ -1396,30 +1396,9 @@ po.arrow = function() {
         }
         break;
       }
-      case 109: case 189: {
-        if (!key.plus) {
-          last = now;
-          key.plus = 1;
-          if (!key.minus) dz = -1;
-        }
-        break;
-      }
-      case 61: case 187: {
-        if (!key.minus) {
-          last = now;
-          key.minus = 1;
-          if (!key.plus) dz = 1;
-        }
-        break;
-      }
       default: return;
     }
-    if (dz) {
-      var z = map.zoom();
-      map.zoom(dz < 0 ? Math.ceil(z) - 1 : Math.floor(z) + 1);
-    } else if (dx || dy) {
-      map.panBy({x: dx, y: dy});
-    }
+    if (dx || dy) map.panBy({x: dx, y: dy});
     if (!repeatTimer && (key.left | key.right | key.up | key.down)) {
       repeatTimer = setInterval(repeat, repeatInterval);
     }
@@ -1433,8 +1412,6 @@ po.arrow = function() {
       case 39: key.right = 0; break;
       case 38: key.up = 0; break;
       case 40: key.down = 0; break;
-      case 109: case 189: key.plus = 0; break;
-      case 61: case 187: key.minus = 0; break;
       default: return;
     }
     if (repeatTimer && !(key.left | key.right | key.up | key.down)) {
@@ -1443,6 +1420,15 @@ po.arrow = function() {
     e.preventDefault();
   }
 
+  function keypress(e) {
+    switch (e.charCode) {
+      case 45: case 95: map.zoom(Math.ceil(map.zoom()) - 1); break;
+      case 43: case 61: map.zoom(Math.floor(map.zoom()) + 1); break;
+      default: return;
+    }
+    e.preventDefault();
+  }
+
   function repeat() {
     if (!map) return;
     if (Date.now() < last + repeatDelay) return;
@@ -1454,12 +1440,14 @@ po.arrow = function() {
   arrow.map = function(x) {
     if (!arguments.length) return map;
     if (map) {
+      parent.removeEventListener("keypress", keypress, false);
       parent.removeEventListener("keydown", keydown, false);
       parent.removeEventListener("keyup", keyup, false);
       parent = null;
     }
     if (map = x) {
       parent = map.focusableParent();
+      parent.addEventListener("keypress", keypress, false);
       parent.addEventListener("keydown", keydown, false);
       parent.addEventListener("keyup", keyup, false);
     }