Bug 1532719 - Reset _pinnedUnscrollable flag when unpinning a tab. r=Gijs

Differential Revision: https://phabricator.services.mozilla.com/D22474

browser/base/content/tabbrowser.js

@@ -667,6 +667,7 @@ window._gBrowser = {
     this.moveTabTo(aTab, this._numPinnedTabs - 1);
     aTab.removeAttribute("pinned");
     aTab.style.marginInlineStart = "";
+    aTab._pinnedUnscrollable = false;
     this._updateTabBarForPinnedTabs();
     this._notifyPinnedStatus(aTab);
     this._maybeUpdateNumPinnedTabsPref();