Fix button tooltip

Summary:
This fixes an issue with D10276808. It turns out that the function returned from `addTooltip()` closes over the node so we can't simply re-call it.

The reason `addTooltip()` normally works at all is *very* subtle. It relies on the fact that the returned value changes each render, causing it to be invoked with `null` and then the same element in succession each time. The cleanup is then based on the time between those two (it sets a timeout each render).

Reviewed By: Goom11

Differential Revision: D10435657

fbshipit-source-id: b487d6050d0b953fb917e72cc5a5b1a6f21eb6d3

modules/nuclide-commons-ui/Button.js

@@ -75,7 +75,7 @@ const ButtonTypeClassnames = Object.freeze({
  */
 export class Button extends React.Component<Props> {
   _button: ?HTMLButtonElement;
-  _lastTooltipOptions: ?atom$TooltipsAddOptions = null;
+  _removeTooltip: ?() => void;
 
   focus(): void {
     const node = ReactDOM.findDOMNode(this);
@@ -94,17 +94,18 @@ export class Button extends React.Component<Props> {
       onButtonDOMNodeChange(this._button);
     }
 
-    // If the element goes away (e.g. on unmount), remove the tooltip.
-    if (button == null && this._lastTooltipOptions != null) {
-      addTooltip(this._lastTooltipOptions)(null);
-      this._lastTooltipOptions = null;
+    // When the element goes away (e.g. on unmount), remove the tooltip.
+    if (button == null && this._removeTooltip != null) {
+      this._removeTooltip();
     }
 
-    if (!disabled) {
-      if (tooltip && !disabled) {
-        addTooltip(tooltip)(button);
-        this._lastTooltipOptions = tooltip;
-      }
+    if (!disabled && tooltip && button != null) {
+      const updateTooltip = addTooltip(tooltip);
+      updateTooltip(button);
+      this._removeTooltip = () => {
+        updateTooltip(null);
+        this._removeTooltip = null;
+      };
     }
   };