Focus center event if scrolled up when ctrl+j/k

When using the focus(Previous/Next)Message keybinds, no message is
focused and the view has been scrolled up, focus the message in the
center of the view instead of returning to the list's bottom to focus
the most recent one.

TODO.md

@@ -3,6 +3,7 @@
 - handle invalid access token
 - If an account is gone from the user's config, discard UI state last page
 - filter > enter > room list is always scrolled to top
+- ctrl+j/k when scrolled up
 - session list: prevent tab-focusing the delegates
 - refresh server list button
 
@@ -292,3 +293,5 @@
 
 - Technology Preview: Support for running Qt Quick (2D) on top of 
   Direct3D, Metal and Vulkan
+
+- `ListView.reuseItems` property

src/gui/Pages/Chat/Timeline/EventList.qml

@@ -26,15 +26,12 @@ Rectangle {
 
     HShortcut {
         sequences: window.settings.keys.focusPreviousMessage
-        onActivated: eventList.incrementCurrentIndex()
+        onActivated: eventList.focusPreviousMessage()
     }
 
     HShortcut {
         sequences: window.settings.keys.focusNextMessage
-        onActivated:
-            eventList.currentIndex === 0 ?
-            eventList.currentIndex = -1 :
-            eventList.decrementCurrentIndex()
+        onActivated: eventList.focusNextMessage()
     }
 
     HShortcut {
@@ -244,6 +241,29 @@ Rectangle {
         readonly property var redactableCheckedEvents:
             getSortedChecked().filter(ev => eventList.canRedact(ev))
 
+        function focusCenterMessage() {
+            const previous     = highlightRangeMode
+            highlightRangeMode = HListView.NoHighlightRange
+            currentIndex       = indexAt(0, contentY + height / 2)
+            highlightRangeMode = previous
+        }
+
+        function focusPreviousMessage() {
+            currentIndex === -1 && contentY < -height - bottomMargin * 2 ?
+            focusCenterMessage() :
+            incrementCurrentIndex()
+        }
+
+        function focusNextMessage() {
+            currentIndex === -1 && contentY <= -height - bottomMargin * 2 ?
+            focusCenterMessage() :
+
+            eventList.currentIndex === 0 ?
+            eventList.currentIndex = -1 :
+
+            decrementCurrentIndex()
+        }
+
         function copySelectedDelegates() {
             if (eventList.selectedText) {
                 Clipboard.text = eventList.selectedText