Reduce horizontal swipe sensitivity in timelines (#3148)

* Reduce horizontal swipe sensitivity in timelines

Fixes https://github.com/tuskyapp/Tusky/issues/2725, fixes https://github.com/tuskyapp/Tusky/issues/2112, fixes https://github.com/tuskyapp/Tusky/issues/2530, fixes https://github.com/tuskyapp/Tusky/issues/2200, fixes https://github.com/tuskyapp/Tusky/issues/2176, fixes https://github.com/tuskyapp/Tusky/issues/2112, fixes https://github.com/tuskyapp/Tusky/issues/1912, fixes https://github.com/tuskyapp/Tusky/issues/1718, fixes https://github.com/tuskyapp/Tusky/issues/1336

* Set scale factor to 4

* Catch exceptions, just in case

app/src/main/java/com/keylesspalace/tusky/MainActivity.kt

@@ -89,6 +89,7 @@ import com.keylesspalace.tusky.util.deleteStaleCachedMedia
 import com.keylesspalace.tusky.util.emojify
 import com.keylesspalace.tusky.util.getDimension
 import com.keylesspalace.tusky.util.hide
+import com.keylesspalace.tusky.util.reduceSwipeSensitivity
 import com.keylesspalace.tusky.util.show
 import com.keylesspalace.tusky.util.updateShortcut
 import com.keylesspalace.tusky.util.viewBinding
@@ -253,6 +254,8 @@ class MainActivity : BottomSheetActivity(), ActionButtonActivity, HasAndroidInje
             }
         }
 
+        binding.viewPager.reduceSwipeSensitivity()
+
         setupDrawer(savedInstanceState, addSearchButton = hideTopToolbar)
 
         /* Fetch user info while we're doing other things. This has to be done after setting up the

app/src/main/java/com/keylesspalace/tusky/components/account/AccountActivity.kt

@@ -78,6 +78,7 @@ import com.keylesspalace.tusky.util.getDomain
 import com.keylesspalace.tusky.util.hide
 import com.keylesspalace.tusky.util.loadAvatar
 import com.keylesspalace.tusky.util.parseAsMastodonHtml
+import com.keylesspalace.tusky.util.reduceSwipeSensitivity
 import com.keylesspalace.tusky.util.setClickableText
 import com.keylesspalace.tusky.util.show
 import com.keylesspalace.tusky.util.viewBinding
@@ -235,6 +236,7 @@ class AccountActivity : BottomSheetActivity(), ActionButtonActivity, HasAndroidI
         // Setup the tabs and timeline pager.
         adapter = AccountPagerAdapter(this, viewModel.accountId)
 
+        binding.accountFragmentViewPager.reduceSwipeSensitivity()
         binding.accountFragmentViewPager.adapter = adapter
         binding.accountFragmentViewPager.offscreenPageLimit = 2
 

app/src/main/java/com/keylesspalace/tusky/components/search/SearchActivity.kt

@@ -30,6 +30,7 @@ import com.keylesspalace.tusky.components.search.adapter.SearchPagerAdapter
 import com.keylesspalace.tusky.databinding.ActivitySearchBinding
 import com.keylesspalace.tusky.di.ViewModelFactory
 import com.keylesspalace.tusky.settings.PrefKeys
+import com.keylesspalace.tusky.util.reduceSwipeSensitivity
 import com.keylesspalace.tusky.util.viewBinding
 import dagger.android.DispatchingAndroidInjector
 import dagger.android.HasAndroidInjector
@@ -62,6 +63,7 @@ class SearchActivity : BottomSheetActivity(), HasAndroidInjector {
     }
 
     private fun setupPages() {
+        binding.pages.reduceSwipeSensitivity()
         binding.pages.adapter = SearchPagerAdapter(this)
 
         val enableSwipeForTabs = preferences.getBoolean(PrefKeys.ENABLE_SWIPE_FOR_TABS, true)

app/src/main/java/com/keylesspalace/tusky/util/ViewExtensions.kt

@@ -18,8 +18,11 @@ package com.keylesspalace.tusky.util
 
 import android.text.Editable
 import android.text.TextWatcher
+import android.util.Log
 import android.view.View
 import android.widget.EditText
+import androidx.recyclerview.widget.RecyclerView
+import androidx.viewpager2.widget.ViewPager2
 
 fun View.show() {
     this.visibility = View.VISIBLE
@@ -63,3 +66,37 @@ inline fun EditText.afterTextChanged(
         }
     })
 }
+
+/**
+ * Reduce ViewPager2's sensitivity to horizontal swipes.
+ */
+fun ViewPager2.reduceSwipeSensitivity() {
+    // ViewPager2 is very sensitive to horizontal motion when swiping vertically, and will
+    // trigger a page transition if the user's swipe is only a few tens of degrees off from
+    // vertical. This is a problem if the underlying content is a list that the user wants
+    // to scroll vertically -- it's far too easy to trigger an accidental horizontal swipe.
+    //
+    // One way to stop this is to reach in to ViewPager2's RecyclerView and adjust the amount
+    // of touch slop it has. Scaling by 2 appears to work well.
+    //
+    // See https://issuetracker.google.com/issues/139867645 and
+    // https://bladecoder.medium.com/fixing-recyclerview-nested-scrolling-in-opposite-direction-f587be5c1a04
+    // for more (the approach in that Medium article works, but is still quite sensitive to
+    // horizontal movement while scrolling).
+    try {
+        val recyclerViewField = ViewPager2::class.java.getDeclaredField("mRecyclerView")
+        recyclerViewField.isAccessible = true
+        val recyclerView = recyclerViewField.get(this) as RecyclerView
+
+        val touchSlopField = RecyclerView::class.java.getDeclaredField("mTouchSlop")
+        touchSlopField.isAccessible = true
+        val touchSlop = touchSlopField.get(recyclerView) as Int
+        // 4 seems to be a sweet-spot. 2-3 still causes a horizontal swipe right if the user drags
+        // down-left at ~ 45 degree angle. Experimentally, 4 requires the swipe to be +/- ~ 10 degrees
+        // from horizontal to register as a horizontal and not a vertical swipe.
+        val scaleFactor = 4
+        touchSlopField.set(recyclerView, touchSlop * scaleFactor)
+    } catch (e: Exception) {
+        Log.w("reduceSwipeSensitivity", e)
+    }
+}