Issue #18535: Do not animate first scroll to position

This looks less that ideal with a grid layout that swings by from the
normal tabs to private tabs.

app/src/main/java/org/mozilla/fenix/tabstray/TabLayoutMediator.kt

@@ -57,8 +57,19 @@ class TabLayoutMediator(
 internal class TabLayoutObserver(
     private val interactor: TabsTrayInteractor
 ) : TabLayout.OnTabSelectedListener {
+
+    private var initialScroll = true
+
     override fun onTabSelected(tab: TabLayout.Tab) {
-        interactor.setCurrentTrayPosition(tab.position)
+        // Do not animate the initial scroll when opening the tabs tray.
+        val animate = if (initialScroll) {
+            initialScroll = false
+            false
+        } else {
+            true
+        }
+
+        interactor.setCurrentTrayPosition(tab.position, animate)
     }
 
     override fun onTabUnselected(tab: TabLayout.Tab) = Unit

app/src/main/java/org/mozilla/fenix/tabstray/TabsTrayFragment.kt

@@ -104,8 +104,8 @@ class TabsTrayFragment : AppCompatDialogFragment(), TabsTrayInteractor {
         }
     }
 
-    override fun setCurrentTrayPosition(position: Int) {
-        tabsTray.currentItem = position
+    override fun setCurrentTrayPosition(position: Int, smoothScroll: Boolean) {
+        tabsTray.setCurrentItem(position, smoothScroll)
     }
 
     override fun navigateToBrowser() {

app/src/main/java/org/mozilla/fenix/tabstray/TabsTrayInteractor.kt

@@ -7,8 +7,11 @@ package org.mozilla.fenix.tabstray
 interface TabsTrayInteractor {
     /**
      * Set the current tray item to the clamped [position].
+     *
+     * @param position The position on the tray to focus.
+     * @param smoothScroll If true, animate the scrolling from the current tab to [position].
      */
-    fun setCurrentTrayPosition(position: Int)
+    fun setCurrentTrayPosition(position: Int, smoothScroll: Boolean)
 
     /**
      * Dismisses the tabs tray and navigates to the browser.

app/src/test/java/org/mozilla/fenix/tabstray/TabLayoutObserverTest.kt

@@ -21,6 +21,21 @@ class TabLayoutObserverTest {
 
         observer.onTabSelected(tab)
 
-        verify { interactor.setCurrentTrayPosition(1) }
+        verify { interactor.setCurrentTrayPosition(1, false) }
+    }
+
+    @Test
+    fun `WHEN observer is first started THEN do not smooth scroll`() {
+        val observer = TabLayoutObserver(interactor)
+        val tab = mockk<TabLayout.Tab>()
+        every { tab.position } returns 1
+
+        observer.onTabSelected(tab)
+
+        verify { interactor.setCurrentTrayPosition(1, false) }
+
+        observer.onTabSelected(tab)
+
+        verify { interactor.setCurrentTrayPosition(1, true) }
     }
 }