Avoid await on promiseDocumentFlushed when the browserAction is placed in the overflow menu

1-Bug_1285500___Fix_browser_ext_browserAction_popup_intermittency_when_browserAction_is_placed_in_the_overflow_menu_.patch

# HG changeset patch
# User Luca Greco <lgreco@mozilla.com>
# Date 1551125488 -3600
#      Mon Feb 25 21:11:28 2019 +0100
# Node ID 42519936d019a00bd9ea52dcb6acf98a7545412b
# Parent  198cd4a81bf2afa7cc79360f90da7bc91218b76d
Bug 1285500 - Fix browser_ext_browserAction_popup intermittency when browserAction is placed in the overflow menu.

diff --git a/browser/components/extensions/ExtensionPopups.jsm b/browser/components/extensions/ExtensionPopups.jsm
--- a/browser/components/extensions/ExtensionPopups.jsm
+++ b/browser/components/extensions/ExtensionPopups.jsm
@@ -537,7 +537,7 @@ class ViewPopup extends BasePopup {
 
     this.setBackground(this.background);
 
-    let flushPromise = this.window.promiseDocumentFlushed(() => {
+    const getSizes = () => {
       let win = this.window;
 
       // Calculate the extra height available on the screen above and below the
@@ -555,7 +555,19 @@ class ViewPopup extends BasePopup {
         bottom: Math.max(0, screenBottom - popupBottom),
         top:  Math.max(0, popupTop - win.screen.availTop),
       };
-    });
+    };
+
+    let flushPromise;
+
+    if (panel.id === "widget-overflow") {
+      // Get the popup sizes synchronously when the browserAction is placed in
+      // the overflow menu
+      getSizes();
+    } else {
+      // Wait for the ChromeWindow document to be flushed before retrieving
+      // the popup sizes when the browserAction is placed in the toolbar;
+      flushPromise = this.window.promiseDocumentFlushed(getSizes);
+    }
 
     // Create a new browser in the real popup.
     let browser = this.browser;
@@ -564,14 +576,16 @@ class ViewPopup extends BasePopup {
     this.browser.swapDocShells(browser);
     this.destroyBrowser(browser);
 
-    await flushPromise;
+    if (flushPromise) {
+      await flushPromise;
 
-    // Check if the popup has been destroyed while we were waiting for the
-    // document flush promise to be resolve.
-    if (this.destroyed) {
-      this.closePopup();
-      this.destroy();
-      return false;
+      // Check if the popup has been destroyed while we were waiting for the
+      // document flush promise to be resolve.
+      if (this.destroyed) {
+        this.closePopup();
+        this.destroy();
+        return false;
+      }
     }
 
     if (this.dimensions) {