Bug 1456056 - Part 2. Display chevron button only if the devtool width is narrow.r=jdescottes

This patch will allow that all buttons which accessing the tool panel is
overflowed. i.e. toolbar will display chevron button only.

MozReview-Commit-ID: GbKbAhtpYt7

--HG--
extra : rebase_source : 971aef121c329e6a5ba3ada24015a1d820aab26a

devtools/client/framework/components/ToolboxTabs.js

@@ -165,11 +165,12 @@ class ToolboxTabs extends Component {
         let removingToolWidth = this._cachedToolTabsWidthMap.get(removingToolId);
         sumWidth -= removingToolWidth;
       }
-      visibleTabs.push(currentToolId);
-    }
 
-    if (visibleTabs.length === 0) {
-      visibleTabs = [enabledTabs[0]];
+      // If toolbox width is narrow, toolbox display only chevron menu.
+      // i.e. All tool tabs will overflow.
+      if ((sumWidth + selectedToolWidth) <= toolboxWidth) {
+        visibleTabs.push(currentToolId);
+      }
     }
 
     let willOverflowTabs = enabledTabs.filter(id => !visibleTabs.includes(id));