--- a/mobile/android/base/java/org/mozilla/gecko/tabs/TabsGridLayout.java
+++ b/mobile/android/base/java/org/mozilla/gecko/tabs/TabsGridLayout.java
@@ -2,26 +2,28 @@
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this file,
  * You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 package org.mozilla.gecko.tabs;
 
 import org.mozilla.gecko.R;
 import org.mozilla.gecko.Tab;
-import org.mozilla.gecko.widget.SpacingDecoration;
+import org.mozilla.gecko.Tabs;
+import org.mozilla.gecko.widget.GridSpacingDecoration;
 
 import android.content.Context;
 import android.content.res.Resources;
 import android.support.v7.widget.DefaultItemAnimator;
 import android.support.v7.widget.GridLayoutManager;
 import android.support.v7.widget.helper.ItemTouchHelper;
 import android.util.AttributeSet;
 
 public class TabsGridLayout extends TabsLayout {
+    private final GridSpacingDecoration spacingDecoration;
 
     public TabsGridLayout(Context context, AttributeSet attrs) {
         super(context, attrs, R.layout.tabs_layout_item_view);
 
         final Resources resources = context.getResources();
 
         setLayoutManager(new GridLayoutManager(context, 1));
         setAutoFit(true);
@@ -29,26 +31,16 @@ public class TabsGridLayout extends Tabs
         final int itemWidth = resources.getDimensionPixelSize(R.dimen.tab_panel_item_width);
         final int horizontalItemPadding = resources.getDimensionPixelSize(R.dimen.tab_panel_grid_item_hpadding);
         final int verticalItemPadding = resources.getDimensionPixelSize(R.dimen.tab_panel_grid_item_vpadding);
         final int viewPaddingHorizontal = resources.getDimensionPixelSize(R.dimen.tab_panel_grid_hpadding);
         final int viewPaddingVertical = resources.getDimensionPixelSize(R.dimen.tab_panel_grid_vpadding);
 
         setDesiredColumnWidth(itemWidth + 2 * horizontalItemPadding);
 
-        // TODO We'll need to be more specific about how much space the ItemDecoration should fill
-        // in - otherwise we're left with empty item space that can be clicked even though it
-        // appears to the user that there should be no content there.
-        // (The issue is that GridLayoutManager left aligns its content in a given column, so we
-        // need to make content full width, and then (until we support setting thumbnail widths on
-        // the fly in the tabs layout) to keep the portion of the content outside the fixed-size
-        // thumbnail/title region unclickable, we need to fill in that region with ItemDecoration,
-        // which is what remains to be done.)
-        addItemDecoration(new SpacingDecoration(horizontalItemPadding, verticalItemPadding));
-
         setPadding(viewPaddingHorizontal, viewPaddingVertical, viewPaddingHorizontal, viewPaddingVertical);
         setClipToPadding(false);
         setScrollBarStyle(SCROLLBARS_OUTSIDE_OVERLAY);
 
         final DefaultItemAnimator animator = new DefaultItemAnimator();
         // On close we only animate the moves, not the remove.
         animator.setRemoveDuration(0);
         // A fade in/out each time the title/thumbnail/etc. gets updated isn't helpful, so disable
@@ -60,16 +52,30 @@ public class TabsGridLayout extends Tabs
         final TabsTouchHelperCallback callback = new TabsTouchHelperCallback(this) {
             @Override
             protected float alphaForItemSwipeDx(float dX, int distanceToAlphaMin) {
                 return 1f - 2f * Math.abs(dX) / distanceToAlphaMin;
             }
         };
         final ItemTouchHelper touchHelper = new ItemTouchHelper(callback);
         touchHelper.attachToRecyclerView(this);
+
+        spacingDecoration = new GridSpacingDecoration(itemWidth, verticalItemPadding);
+        addItemDecoration(spacingDecoration);
+    }
+
+    @Override
+    protected void onSpanCountChanged() {
+        // spanCount can change (in onMeasure) at a point where some ItemDecorations have been
+        // computed and some have not, and Android doesn't recompute ItemDecorations after a
+        // setSpanCount call, so we need to remove and then add back our spacingDecoration (whose
+        // computations depend on spanCount) in order to get a full layout recompute.
+        removeItemDecoration(spacingDecoration);
+        addItemDecoration(spacingDecoration);
+        updateSelectedPosition();
     }
 
     @Override
     public void closeAll() {
         autoHidePanel();
 
         closeAllTabs();
     }

--- a/mobile/android/base/java/org/mozilla/gecko/tabs/TabsLayout.java
+++ b/mobile/android/base/java/org/mozilla/gecko/tabs/TabsLayout.java
@@ -144,17 +144,17 @@ public abstract class TabsLayout extends
         final int tabId = item.getTabId();
         final Tabs tabs = Tabs.getInstance();
         tabs.selectTab(tabId);
         autoHidePanel();
         tabs.notifyListeners(tabs.getTab(tabId), Tabs.TabEvents.OPENED_FROM_TABS_TRAY);
     }
 
     // Updates the selected position in the list so that it will be scrolled to the right place.
-    private void updateSelectedPosition() {
+    protected void updateSelectedPosition() {
         final int selected = getSelectedAdapterPosition();
         if (selected != NO_POSITION) {
             scrollToPosition(selected);
         }
     }
 
     private void refreshTabsData() {
         // Store a different copy of the tabs, so that we don't have to worry about

new file mode 100644
--- /dev/null
+++ b/mobile/android/base/java/org/mozilla/gecko/widget/GridSpacingDecoration.java
@@ -0,0 +1,68 @@
+/* -*- Mode: Java; c-basic-offset: 4; tab-width: 20; indent-tabs-mode: nil; -*-
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+package org.mozilla.gecko.widget;
+
+import android.graphics.Rect;
+import android.support.v7.widget.GridLayoutManager;
+import android.support.v7.widget.RecyclerView;
+import android.view.View;
+
+// An ItemDecoration for a GridLayoutManager to adjust fixed width items within their columns so as
+// to maintain as much space between items as possible.  The computations take into account and use
+// any horizontal RecyclerView padding as part of the spacing of first and last items in a row.
+// This decoration assumes the RecyclerView's left and right padding are equal.
+public class GridSpacingDecoration extends RecyclerView.ItemDecoration {
+    private final int verticalPadding;
+    private final int itemWidth;
+
+    public GridSpacingDecoration(int itemWidth, int verticalPadding) {
+        this.verticalPadding = verticalPadding;
+        this.itemWidth = itemWidth;
+    }
+
+    @Override
+    public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) {
+        final int position = parent.getChildAdapterPosition(view);
+        if (position == RecyclerView.NO_POSITION) {
+            return;
+        }
+
+        final GridLayoutManager layoutManager = (GridLayoutManager) parent.getLayoutManager();
+        final int spanCount = layoutManager.getSpanCount();
+        final int layoutWidth = parent.getWidth();
+        // We currently assume left and right padding are equal.
+        final int layoutHorizontalPadding = layoutManager.getPaddingLeft();
+        final int innerLayoutWidth = layoutWidth - 2 * layoutHorizontalPadding;
+
+        if (spanCount == 1) {
+            final int offset = (innerLayoutWidth - itemWidth) / 2;
+            outRect.set(offset, verticalPadding, offset, verticalPadding);
+            return;
+        }
+
+        // The formulas here compute the start and end of each item within its column so as to
+        // evenly space items within the inner layout (i.e. the space between the layout padding)
+        // while still using the layout padding as part of the outer spacing of the outer items to
+        // the extent possible.
+        final int rowPosition = position % spanCount;
+        // The ideal spacing has equal spacing (on each side) around every item...
+        final int idealSpacing = (layoutWidth - spanCount * itemWidth) / (spanCount + 1);
+        // ...but we're confined to having at least the layout padding's amount of spacing on the
+        // outer edges of the outer items, so use that as much as possible, and then compute
+        // outerOffset to be how much more (if any) is needed for the outer edge of the outer items.
+        final int outerOffset = Math.max(0, idealSpacing - layoutHorizontalPadding);
+        // Offset for the start of this view from the inner edge of the left padding:
+        final float innerLayoutOffset =
+                rowPosition * ((float) (innerLayoutWidth - 2 * outerOffset - itemWidth) / (float) (spanCount - 1)) + outerOffset;
+        // Left offset of the start of this view within its column:
+        final int columnLeftOffset = (int) (innerLayoutOffset - (float) (rowPosition * innerLayoutWidth) / (float) spanCount);
+        // Right offset of the end of this view within its column:
+        final int columnRightOffset =
+                (int) ((float) ((rowPosition + 1) * innerLayoutWidth) / (float) spanCount - (innerLayoutOffset + itemWidth));
+
+        outRect.set(columnLeftOffset, verticalPadding, columnRightOffset, verticalPadding);
+    }
+}

--- a/mobile/android/base/moz.build
+++ b/mobile/android/base/moz.build
@@ -738,16 +738,17 @@ gbjar.sources += ['java/org/mozilla/geck
     'widget/FadedMultiColorTextView.java',
     'widget/FadedSingleColorTextView.java',
     'widget/FadedTextView.java',
     'widget/FaviconView.java',
     'widget/FilledCardView.java',
     'widget/FlowLayout.java',
     'widget/GeckoActionProvider.java',
     'widget/GeckoPopupMenu.java',
+    'widget/GridSpacingDecoration.java',
     'widget/HistoryDividerItemDecoration.java',
     'widget/IconTabWidget.java',
     'widget/LoginDoorHanger.java',
     'widget/RecyclerViewClickSupport.java',
     'widget/ResizablePathDrawable.java',
     'widget/RoundedCornerLayout.java',
     'widget/SiteLogins.java',
     'widget/SpacingDecoration.java',