Use a scrollable tab strip on tablets at small widths

chrome/android/java/src/org/chromium/chrome/browser/compositor/overlays/strip/StripLayoutHelper.java

Index: chrome/android/java/src/org/chromium/chrome/browser/compositor/overlays/strip/StripLayoutHelper.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/compositor/overlays/strip/StripLayoutHelper.java b/chrome/android/java/src/org/chromium/chrome/browser/compositor/overlays/strip/StripLayoutHelper.java
index dd894353c49ee2f686c92708a82ae5aa5095b8fe..3d7c5edea3120e615ffcadc0b6d9179a4d254c87 100644
--- a/chrome/android/java/src/org/chromium/chrome/browser/compositor/overlays/strip/StripLayoutHelper.java
+++ b/chrome/android/java/src/org/chromium/chrome/browser/compositor/overlays/strip/StripLayoutHelper.java
@@ -30,7 +30,6 @@ import org.chromium.chrome.browser.compositor.layouts.LayoutRenderHost;
 import org.chromium.chrome.browser.compositor.layouts.LayoutUpdateHost;
 import org.chromium.chrome.browser.compositor.layouts.components.CompositorButton;
 import org.chromium.chrome.browser.compositor.layouts.components.VirtualView;
-import org.chromium.chrome.browser.compositor.layouts.content.TabContentManager;
 import org.chromium.chrome.browser.compositor.layouts.phone.stack.StackScroller;
 import org.chromium.chrome.browser.compositor.overlays.strip.TabLoadTracker.TabLoadTrackerCallback;
 import org.chromium.chrome.browser.tab.Tab;
@@ -38,6 +37,7 @@ import org.chromium.chrome.browser.tabmodel.TabCreatorManager.TabCreator;
 import org.chromium.chrome.browser.tabmodel.TabModel;
 import org.chromium.chrome.browser.tabmodel.TabModelUtils;
 import org.chromium.chrome.browser.util.MathUtils;
+import org.chromium.ui.base.DeviceFormFactor;
 import org.chromium.ui.base.LocalizationUtils;
 
 import java.util.ArrayList;
@@ -67,7 +67,7 @@ public class StripLayoutHelper {
     // Animation/Timer Constants
     private static final int RESIZE_DELAY_MS = 1500;
     private static final int SPINNER_UPDATE_DELAY_MS = 66;
-    // Degrees per milisecond.
+    // Degrees per millisecond.
     private static final float SPINNER_DPMS = 0.33f;
     private static final int EXPAND_DURATION_MS = 250;
     private static final int ANIM_TAB_CREATED_MS = 150;
@@ -88,6 +88,7 @@ public class StripLayoutHelper {
     private static final float NEW_TAB_BUTTON_CLICK_SLOP_DP = 4.f;
     private static final float NEW_TAB_BUTTON_WIDTH_DP = 58.f;
     private static final float NEW_TAB_BUTTON_HEIGHT_DP = 32.5f;
+    static final float FADE_FULL_OPACITY_THRESHOLD_DP = 24.f;
 
     private static final int MESSAGE_RESIZE = 1;
     private static final int MESSAGE_UPDATE_SPINNER = 2;
@@ -97,8 +98,9 @@ public class StripLayoutHelper {
     private final LayoutRenderHost mRenderHost;
     private TabModel mModel;
     private TabCreator mTabCreator;
-    private TabContentManager mTabContentManager;
-    private StripStacker mStripStacker = new StaticStripStacker();
+    private StripStacker mStripStacker;
+    private CascadingStripStacker mCascadingStripStacker = new CascadingStripStacker();
+    private ScrollingStripStacker mScrollingStripStacker = new ScrollingStripStacker();
 
     // Internal State
     private StripLayoutTab[] mStripTabs = new StripLayoutTab[0];
@@ -111,9 +113,8 @@ public class StripLayoutHelper {
     private final CompositorButton mNewTabButton;
 
     // Layout Constants
-    private final float mTabStackWidth;
     private final float mTabOverlapWidth;
-    private float mNewTabButtonWidth;
+    private final float mNewTabButtonWidth;
     private final float mMinTabWidth;
     private final float mMaxTabWidth;
     private final float mReorderMoveStartThreshold;
@@ -141,6 +142,8 @@ public class StripLayoutHelper {
     private float mRightMargin;
     private final boolean mIncognito;
     private float mBrightness;
+    // Whether the CascadingStripStacker should be used.
+    private boolean mShouldCascadeTabs;
 
     // Tab menu item IDs
     public static final int ID_CLOSE_ALL_TABS = 0;
@@ -155,14 +158,9 @@ public class StripLayoutHelper {
      */
     public StripLayoutHelper(Context context, LayoutUpdateHost updateHost,
             LayoutRenderHost renderHost, boolean incognito) {
-        mTabStackWidth = TAB_STACK_WIDTH_DP;
         mTabOverlapWidth = TAB_OVERLAP_WIDTH_DP;
         mNewTabButtonWidth = NEW_TAB_BUTTON_WIDTH_DP;
 
-        if (LocalizationUtils.isLayoutRtl()) {
-            // In rtl let the tab nest closer to the new tab button.
-            mNewTabButtonWidth -= mTabOverlapWidth / 2;
-        }
         mRightMargin = LocalizationUtils.isLayoutRtl() ? 0 : mNewTabButtonWidth;
         mLeftMargin = LocalizationUtils.isLayoutRtl() ? mNewTabButtonWidth : 0;
         mMinTabWidth = MIN_TAB_WIDTH_DP;
@@ -206,6 +204,9 @@ public class StripLayoutHelper {
         int menuWidth = mContext.getResources().getDimensionPixelSize(R.dimen.menu_width);
         mTabMenu.setWidth(menuWidth);
         mTabMenu.setModal(true);
+
+        mShouldCascadeTabs = true;
+        mStripStacker = mCascadingStripStacker;

[Changwan Ryu, 2016/04/06 02:56:43]

Hmm... Can we set the correct value from the beginning?
In case of starting width < 600, we may see flickering in some situation.

[Theresa, 2016/04/06 06:49:51]

Done. Also added logic to handle auto-scrolling when a tab is selected and a
TODO note to add logic for auto-scrolling on cold start and when the tab model
scrolls.

     }
 
     /**
@@ -263,6 +264,47 @@ public class StripLayoutHelper {
     }
 
     /**
+     * @return The opacity to use for the fade on the left side of the tab strip.
+     */
+    public float getLeftFadeOpacity() {
+        return getFadeOpacity(true);
+    }
+
+    /**
+     * @return The opacity to use for the fade on the right side of the tab strip.
+     */
+    public float getRightFadeOpacity() {
+        return getFadeOpacity(false);
+    }
+
+    /**
+     * When the {@link ScrollingStripStacker} is being used, a fade is shown at the left and
+     * right edges to indicate there is tab strip content off screen. As the scroll position
+     * approaches the edge of the screen, the fade opacity is lowered.
+     *
+     * @param isLeft Whether the opacity for the left or right side should be returned.
+     * @return The opacity to use for the fade.
+     */
+    private float getFadeOpacity(boolean isLeft) {
+        if (mShouldCascadeTabs) return 0.f;
+
+        // In RTL, scroll position 0 is on the right side of the screen, whereas in LTR scroll
+        // position 0 is on the left. Account for that in the offset calculation.
+        boolean isRtl = LocalizationUtils.isLayoutRtl();
+        boolean useUnadjustedScrollOffset = isRtl != isLeft;
+        float offset = -(useUnadjustedScrollOffset ? mScrollOffset
+                : (mMinScrollOffset - mScrollOffset));
+
+        if (offset == 0.f) {
+            return 0.f;
+        } else if (offset >= FADE_FULL_OPACITY_THRESHOLD_DP) {
+            return 1.f;
+        } else {
+            return offset / FADE_FULL_OPACITY_THRESHOLD_DP;
+        }
+    }
+
+    /**
      * Allows changing the visual behavior of the tabs in this stack, as specified by
      * {@code stacker}.
      * @param stacker The {@link StripStacker} that should specify how the tabs should be
@@ -279,21 +321,14 @@ public class StripLayoutHelper {
     }
 
     /**
-     * @parm margin The width of the distance between the left edge of
-     *              the screen and first tab.
+     * @param margin The distance between the last tab and the edge of the screen.
      */
-    public void setLeftMargin(float margin) {
-        mLeftMargin = margin;
-        mLeftMargin += LocalizationUtils.isLayoutRtl() ? mNewTabButtonWidth : 0;
-    }
-
-    /**
-     * @param margin The distance between the rightmost tab and the edge of the
-     *               screen.
-     */
-    public void setRightMargin(float margin) {
-        mRightMargin = margin;
-        mRightMargin += LocalizationUtils.isLayoutRtl() ? 0 : mNewTabButtonWidth;
+    public void setEndMargin(float margin) {
+        if (LocalizationUtils.isLayoutRtl()) {
+            mLeftMargin = margin + mNewTabButtonWidth;
+        } else {
+            mRightMargin = margin + mNewTabButtonWidth;
+        }
     }
 
     /**
@@ -313,7 +348,10 @@ public class StripLayoutHelper {
             mStripTabs[i].setHeight(mHeight);
         }
 
-        if (widthChanged) computeAndUpdateTabWidth(false);
+        if (widthChanged) {
+            computeAndUpdateTabWidth(false);
+            setShouldCascadeTabs(width >= DeviceFormFactor.MINIMUM_TABLET_WIDTH_DP);
+        }
         if (mStripTabs.length > 0) mUpdateHost.requestUpdate();
 
         // Dismiss tab menu, similar to how the app menu is dismissed on orientation change
@@ -321,6 +359,74 @@ public class StripLayoutHelper {
     }
 
     /**
+     * Should be called when the viewport width crosses the 600dp threshold. The
+     * {@link CascadingStripStacker} should be used at 600dp+, otherwise the
+     * {@link ScrollingStripStacker} should be used.
+     * @param shouldCascadeTabs Whether the {@link CascadingStripStacker} should be used.
+     */
+    void setShouldCascadeTabs(boolean shouldCascadeTabs) {
+        if (shouldCascadeTabs != mShouldCascadeTabs) {
+            mShouldCascadeTabs = shouldCascadeTabs;
+            setTabStacker(shouldCascadeTabs ? mCascadingStripStacker : mScrollingStripStacker);
+
+            // Scroll to make the selected tab visible and nearby tabs visible.
+            if (mModel.getTabAt(mModel.index()) != null) {
+                updateScrollOffsetLimits();
+                StripLayoutTab tab = findTabById(mModel.getTabAt(mModel.index()).getId());
+                float delta = calculateOffsetToMakeTabVisible(tab, true, true, true);
+                // During this resize, mMinScrollOffset will be changing, so the scroll effect
+                // cannot be properly animated. Jump to the new scroll offset instead.
+                mScrollOffset += delta;
+            }
+
+            updateStrip();
+        }
+    }
+
+    /**
+     * @return Whether the {@link CascadingStripStacker} is being used.
+     */
+    @VisibleForTesting
+    boolean shouldCascadeTabs() {
+        return mShouldCascadeTabs;
+    }
+
+    /**
+     * @return The with of the tab strip.
+     */
+    @VisibleForTesting
+    float getWidth() {
+        return mWidth;
+    }
+
+    /**
+     * @return The strip's current scroll offset.
+     */
+    @VisibleForTesting
+    int getScrollOffset() {
+        return mScrollOffset;
+    }
+
+    /**
+     * @return The strip's minimum scroll offset.
+     */
+    @VisibleForTesting
+    float getMinimumScrollOffset() {
+        return mMinScrollOffset;
+    }
+
+    /**
+     * Set the scroll offset. Should only be used for testing.
+     * @param scrollOffset The scroll offset.
+     */
+    @VisibleForTesting
+    void setScrollOffsetForTesting(int scrollOffset) {
+        mScrollOffset = scrollOffset;
+        updateStrip();
+    }
+
+
+    /**
      * Updates all internal resources and dimensions.
      * @param context The current Android {@link Context}.
      */
@@ -343,11 +449,11 @@ public class StripLayoutHelper {
     /**
      * Sets the {@link TabModel} that this {@link StripLayoutHelper} will visually represent.
      * @param model The {@link TabModel} to visually represent.
+     * @param tabCreator The {@link TabCreator}, used to create new tabs.
      */
-    public void setTabModel(TabModel model, TabContentManager manager, TabCreator tabCreator) {
+    public void setTabModel(TabModel model, TabCreator tabCreator) {
         if (mModel == model) return;
         mModel = model;
-        mTabContentManager = manager;
         mTabCreator = tabCreator;
         computeAndUpdateTabOrders(false);
     }
@@ -361,7 +467,7 @@ public class StripLayoutHelper {
     public boolean updateLayout(long time, long dt) {
         PerfTraceEvent.instant("StripLayoutHelper:updateLayout");
         final boolean doneAnimating = onUpdateAnimation(time, false);
-        updateStrip(time, dt);
+        updateStrip();
         return doneAnimating;
     }
 
@@ -443,16 +549,32 @@ public class StripLayoutHelper {
         // 3. Figure out which tab needs to be visible.
         StripLayoutTab fastExpandTab = findTabById(prevId);
         boolean allowLeftExpand = false;
+        boolean canExpandSelectedTab = false;
         if (!selected) {
             fastExpandTab = tab;
             allowLeftExpand = true;
         }
 
+        if (!mShouldCascadeTabs) {
+            fastExpandTab = tab;
+            allowLeftExpand = true;
+            canExpandSelectedTab = true;
+        }
+
         // 4. Scroll the stack so that the fast expand tab is visible.
         if (fastExpandTab != null) {
-            float delta =
-                    calculateOffsetToMakeTabVisible(fastExpandTab, false, allowLeftExpand, true);
-            if (delta != 0.f) {
+            float delta = calculateOffsetToMakeTabVisible(
+                    fastExpandTab,
+                    canExpandSelectedTab,
+                    allowLeftExpand,
+                    true);
+
+            // If the ScrollingStripStacker is being used and the new tab button is visible, go
+            // directly to the new scroll offset rather than animating. Animating the scroll causes
+            // the new tab button to disappear for a frame.
+            if (!mShouldCascadeTabs && mNewTabButton.isVisible()) {
+                mScrollOffset += delta;
+            } else if (delta != 0.f) {
                 mScroller.startScroll(mScrollOffset, 0, (int) delta, 0, time, EXPAND_DURATION_MS);
             }
         }
@@ -546,9 +668,16 @@ public class StripLayoutHelper {
             // 2.b. Still scrolling, update the scroll destination here.
             mScroller.setFinalX((int) (mScroller.getFinalX() + deltaX));
         } else {
-            // 2.c. Not scrolling.  Check if we need to fast expand.
-            float fastExpandDelta =
-                    calculateOffsetToMakeTabVisible(mInteractingTab, true, true, true);
+            // 2.c. Not scrolling. Check if we need to fast expand.
+            float fastExpandDelta;
+            if (mShouldCascadeTabs) {
+                fastExpandDelta =
+                        calculateOffsetToMakeTabVisible(mInteractingTab, true, true, true);
+            } else {
+                // Non-cascaded tabs are never hidden behind each other, so there's no need to fast
+                // expand.
+                fastExpandDelta = 0.f;
+            }
 
             if (mInteractingTab != null && fastExpandDelta != 0.f) {
                 if ((fastExpandDelta > 0 && deltaX > 0) || (fastExpandDelta < 0 && deltaX < 0)) {
@@ -874,9 +1003,15 @@ public class StripLayoutHelper {
 
         // 2. Compute the effective width of every tab.
         float tabsWidth = 0.f;
-        for (int i = 0; i < mStripTabs.length; i++) {
-            final StripLayoutTab tab = mStripTabs[i];
-            tabsWidth += (tab.getWidth() - mTabOverlapWidth) * tab.getWidthWeight();
+        if (mShouldCascadeTabs) {
+            for (int i = 0; i < mStripTabs.length; i++) {
+                final StripLayoutTab tab = mStripTabs[i];
+                tabsWidth += (tab.getWidth() - mTabOverlapWidth) * tab.getWidthWeight();
+            }
+        } else {
+            // When tabs aren't cascaded, they're non-animating width weight is always 1.0 so it
+            // doesn't need to be included in this calculation.
+            tabsWidth = mStripTabs.length * (mCachedTabWidth - mTabOverlapWidth);
         }
 
         // 3. Correct fencepost error in tabswidth;
@@ -994,7 +1129,7 @@ public class StripLayoutHelper {
         }
     }
 
-    private void updateStrip(long time, long dt) {
+    private void updateStrip() {
         if (mModel == null) return;
 
         // TODO(dtrainor): Remove this once tabCreated() is refactored to be called even from
@@ -1010,10 +1145,12 @@ public class StripLayoutHelper {
         computeTabInitialPositions();
 
         // 3. Calculate the tab stacking.
-        computeTabOffsetHelper();
+        mStripStacker.setTabOffsets(mModel.index(), mStripTabs, TAB_STACK_WIDTH_DP,
+                MAX_TABS_TO_STACK, mTabOverlapWidth, mLeftMargin, mRightMargin, mWidth,
+                mInReorderMode);
 
         // 4. Calculate which tabs are visible.
-        mStripStacker.performOcclusionPass(mModel.index(), mStripTabs);
+        mStripStacker.performOcclusionPass(mModel.index(), mStripTabs, mWidth);
 
         // 5. Create render list.
         createRenderList();
@@ -1052,91 +1189,6 @@ public class StripLayoutHelper {
         }
     }
 
-    private void computeTabOffsetHelper() {
-        final int selIndex = mModel.index();
-
-        // 1. Calculate the size of the selected tab.  This is used later to figure out how
-        // occluded the tabs are.
-        final StripLayoutTab selTab = selIndex >= 0 ? mStripTabs[selIndex] : null;
-        final float selTabWidth = selTab != null ? selTab.getWidth() : 0;
-        final float selTabVisibleSize = selTabWidth - mTabStackWidth - mTabOverlapWidth;
-
-        for (int i = 0; i < mStripTabs.length; i++) {
-            StripLayoutTab tab = mStripTabs[i];
-
-            float posX = tab.getIdealX();
-
-            // 2. Calculate how many tabs are stacked on the left or the right, giving us an idea
-            // of where we can stack this current tab.
-            int leftStackCount = (i < selIndex) ? Math.min(i, MAX_TABS_TO_STACK)
-                                                : Math.min(MAX_TABS_TO_STACK, selIndex)
-                            + Math.min(MAX_TABS_TO_STACK, i - selIndex);
-
-            int rightStackCount = (i >= selIndex)
-                    ? Math.min(mStripTabs.length - 1 - i, MAX_TABS_TO_STACK)
-                    : Math.min(mStripTabs.length - 1 - selIndex, MAX_TABS_TO_STACK)
-                            + Math.min(selIndex - i, MAX_TABS_TO_STACK);
-
-            if (LocalizationUtils.isLayoutRtl()) {
-                int oldLeft = leftStackCount;
-                leftStackCount = rightStackCount;
-                rightStackCount = oldLeft;
-            }
-
-            // 3. Calculate the proper draw position for the tab.  Clamp based on stacking
-            // rules.
-            float minDrawX = mTabStackWidth * leftStackCount + mLeftMargin;
-            float maxDrawX = mWidth - mTabStackWidth * rightStackCount - mRightMargin;
-
-            float drawX =
-                    MathUtils.clamp(posX + tab.getOffsetX(), minDrawX, maxDrawX - tab.getWidth());
-
-            // TODO(dtrainor): Don't set drawX if the tab is closing?
-            tab.setDrawX(drawX);
-            tab.setDrawY(tab.getOffsetY());
-
-            // 4. Calculate how visible this tab is.
-            float visiblePercentage = 1.f;
-            if (i != selIndex) {
-                final float effectiveTabWidth = Math.max(tab.getWidth(), 1.f);
-                final boolean leftStack =
-                        LocalizationUtils.isLayoutRtl() ? i > selIndex : i < selIndex;
-                final float minVisible = !leftStack ? minDrawX + selTabVisibleSize : minDrawX;
-                final float maxVisible = leftStack ? maxDrawX - selTabVisibleSize : maxDrawX;
-
-                final float clippedTabWidth =
-                        Math.min(posX + effectiveTabWidth, maxVisible) - Math.max(posX, minVisible);
-                visiblePercentage = MathUtils.clamp(clippedTabWidth / effectiveTabWidth, 0.f, 1.f);
-            }
-            tab.setVisiblePercentage(visiblePercentage);
-
-            // 5. Calculate which index we start sliding content for.
-            // When reordering, we don't want to slide the content of the adjacent tabs.
-            int contentOffsetIndex = mInReorderMode ? selIndex + 1 : selIndex;
-
-            // 6. Calculate how much the tab is overlapped on the left side or right for RTL.
-            float hiddenAmount = 0.f;
-            if (i > contentOffsetIndex && i > 0 && mStripStacker.canSlideTitleText()) {
-                // 6.a. Get the effective right edge of the previous tab.
-                final StripLayoutTab prevTab = mStripTabs[i - 1];
-                final float prevLayoutWidth =
-                        (prevTab.getWidth() - mTabOverlapWidth) * prevTab.getWidthWeight();
-                float prevTabRight = prevTab.getDrawX();
-                if (!LocalizationUtils.isLayoutRtl()) prevTabRight += prevLayoutWidth;
-
-                // 6.b. Subtract our current draw X from the previous tab's right edge and
-                // get the percentage covered.
-                hiddenAmount = Math.max(prevTabRight - drawX, 0);
-                if (LocalizationUtils.isLayoutRtl()) {
-                    // Invert The amount because we're RTL.
-                    hiddenAmount = prevLayoutWidth - hiddenAmount;
-                }
-            }
-
-            tab.setContentOffsetX(hiddenAmount);
-        }
-    }
-
     private void createRenderList() {
         // 1. Figure out how many tabs will need to be rendered.
         int renderCount = 0;
@@ -1164,28 +1216,21 @@ public class StripLayoutHelper {
             mNewTabButton.setVisible(false);
             return;
         }
-        mNewTabButton.setVisible(true);
 
-        float leftEdge = mWidth - mRightMargin;
-        float rightEdge = mLeftMargin;
+        // 1. Get offset from strip stacker.
+        float offset = mStripStacker.computeNewTabButtonOffset(mStripTabs,
+                mTabOverlapWidth, mLeftMargin, mRightMargin, mWidth, mNewTabButtonWidth);
 
-        for (int i = 0; i < mStripTabs.length; i++) {
-            StripLayoutTab tab = mStripTabs[i];
-            float layoutWidth = (tab.getWidth() - mTabOverlapWidth) * tab.getWidthWeight();
-            rightEdge = Math.max(tab.getDrawX() + layoutWidth, rightEdge);
-            leftEdge = Math.min(tab.getDrawX(), leftEdge);
+        // 2. Hide the new tab button if it's not visible on the screen.
+        boolean isRtl = LocalizationUtils.isLayoutRtl();
+        if ((isRtl && offset + mNewTabButtonWidth < 0) || (!isRtl && offset > mWidth)) {
+            mNewTabButton.setVisible(false);
+            return;
         }
-        rightEdge = Math.min(rightEdge + mTabOverlapWidth, mWidth - mRightMargin);
-        leftEdge = Math.max(leftEdge, mLeftMargin);
-
-        rightEdge -= mTabOverlapWidth / 2;
+        mNewTabButton.setVisible(true);
 
         // 3. Position the new tab button.
-        if (!LocalizationUtils.isLayoutRtl()) {
-            mNewTabButton.setX(rightEdge);
-        } else {
-            mNewTabButton.setX(leftEdge - mNewTabButtonWidth);
-        }
+        mNewTabButton.setX(offset);
     }
 
     private float calculateOffsetToMakeTabVisible(StripLayoutTab tab, boolean canExpandSelectedTab,
@@ -1219,10 +1264,16 @@ public class StripLayoutHelper {
 
         // 4. Return the proper deltaX that has to be applied to the current scroll to see the
         // tab.
-        if (mScrollOffset < optimalLeft && canExpandLeft) {
+        if (mShouldCascadeTabs) {
+            if (mScrollOffset < optimalLeft && canExpandLeft) {
+                return optimalLeft - mScrollOffset;
+            } else if (mScrollOffset > optimalRight && canExpandRight) {
+                return optimalRight - mScrollOffset;
+            }
+        } else {
+            // If tabs are not cascaded, the entire tab strip scrolls and the strip should be
+            // scrolled to the optimal left offset.
             return optimalLeft - mScrollOffset;
-        } else if (mScrollOffset > optimalRight && canExpandRight) {
-            return optimalRight - mScrollOffset;
         }
 
         // 5. We don't have to do anything.  Return no delta.
@@ -1286,9 +1337,14 @@ public class StripLayoutHelper {
         TabModelUtils.setIndex(
                 mModel, TabModelUtils.getTabIndexById(mModel, mInteractingTab.getId()));
 
-        // 5. Fast expand to make sure this tab is visible.
-        float fastExpandDelta = calculateOffsetToMakeTabVisible(mInteractingTab, true, true, true);
-        mScroller.startScroll(mScrollOffset, 0, (int) fastExpandDelta, 0, time, EXPAND_DURATION_MS);
+        // 5. Fast expand to make sure this tab is visible. If tabs are not cascaded, the selected
+        //    tab will already be visible, so there's no need to fast expand to make it visible.
+        if (mShouldCascadeTabs) {
+            float fastExpandDelta =
+                    calculateOffsetToMakeTabVisible(mInteractingTab, true, true, true);
+            mScroller.startScroll(mScrollOffset, 0, (int) fastExpandDelta, 0, time,
+                    EXPAND_DURATION_MS);
+        }
 
         // 6. Request an update.
         mUpdateHost.requestUpdate();