always expand the omnibar when top of the page is reached (#5968)

Task/Issue URL:
https://app.asana.com/1/137249556945/project/1208671518894266/task/1210004588627880?focus=true

### Description
Fixes an issue with the omnibar not showing up when scrolling up by
ensuring we always expand it when top of the page is reached. Details in
the task.

### Steps to test this PR

- [ ] Play around with the omnibar and ensure that it's always shown
when top of the page is reached, even when it's via a fling gesture.
- [ ] Try bottom omnibar too.

You can also try scrolling up slowly and check for any jitter - should
be none. However, if you creep up very very slowly to the top of the
page, without flinging and without allowing the omnibar to expand, it
won't automatically expand to prevent it being misinterpreted as a
gesture in the opposite direction and causing unintended content shift.

Author: LukasPaczos

app/src/main/java/com/duckduckgo/app/browser/BrowserTabFragment.kt

@@ -623,6 +623,7 @@ class BrowserTabFragment :
         get() = activity as? BrowserActivity
 
     private var webView: DuckDuckGoWebView? = null
+    private var isWebViewGestureInProgress = false
 
     private val tabSwitcherActivityResult = registerForActivityResult(StartActivityForResult()) { result ->
         if (result.resultCode == RESULT_OK) {
@@ -2853,12 +2854,30 @@ class BrowserTabFragment :
             }
 
             it.setOnTouchListener { webView, event ->
+                when (event.actionMasked) {
+                    MotionEvent.ACTION_DOWN -> {
+                        isWebViewGestureInProgress = true
+                    }
+
+                    MotionEvent.ACTION_UP, MotionEvent.ACTION_CANCEL -> {
+                        isWebViewGestureInProgress = false
+                    }
+                }
+
                 if (omnibar.omnibarTextInput.isFocused) {
                     binding.focusDummy.requestFocus()
                 }
                 dismissAppLinkSnackBar()
                 false
             }
+            it.setOnScrollChangeListener { v, _, _, _, _ ->
+                if (!v.canScrollVertically(-1) && !isWebViewGestureInProgress) {
+                    // Automatically expand the omnibar when the web view is at the top, but only if the user isn't actively scrolling.
+                    // Expanding the omnibar changes the web view's offset, which could otherwise be misinterpreted by the framework
+                    // as a fling gesture in the opposite direction and cause unintended content shifting.
+                    omnibar.setExpanded(true)
+                }
+            }
 
             it.setEnableSwipeRefreshCallback { enable ->
                 binding.swipeRefreshContainer?.isEnabled = enable