transcriptview perf: only draw the items that are actually visible.

src/transcript/transcript-drawing-area.vala

@@ -4,12 +4,25 @@ using Gee;
 private class TranscriptDrawingArea : Widget 
 {
     public bool show_sender = true;
+    public Adjustment? viewport {
+        get {
+            return _viewport;
+        }
+
+        set {
+            _viewport = value;
+            queue_draw();
+        }
+    }
 
     private ArrayList<Message> _messages = new ArrayList<Message>();
     private ArrayList<ChatItemLayout> _chat_items = new ArrayList<ChatItemLayout>();
 
+    private Adjustment? _viewport = null;
     private const float bubble_margin = 18.0f;
 
+    private const bool debug_viewport = false;
+
     public TranscriptDrawingArea() {
         add_css_class("transcript-drawing-area");
     }
@@ -51,33 +64,63 @@ private class TranscriptDrawingArea : Widget
     }
 
     public override void snapshot(Snapshot snapshot) {
-        float y_offset = 0;
-        var container_width = get_width();
+        const int viewport_y_padding = 50;
+        var viewport_rect = Graphene.Rect() {
+            origin = Graphene.Point() { x = 0, y = ((int)viewport.value - viewport_y_padding) },
+            size = Graphene.Size() { width = get_width(), height = (int)viewport.page_size + (viewport_y_padding * 2) }
+        };
+
+        if (debug_viewport) {
+            // Draw viewport outline for debugging
+            var color = Gdk.RGBA() { red = 1.0f, green = 0, blue = 0, alpha = 1.0f };
+            snapshot.append_border(
+                Gsk.RoundedRect().init_from_rect(viewport_rect, 0),
+                { 1, 1, 1, 1 },
+                { color, color, color, color }
+            );
+        }
 
         // Draw each item in reverse order, since the messages are in reverse order
+        float y_offset = 0;
+        int container_width = get_width();
         for (int i = _chat_items.size - 1; i >= 0; i--) {
             var chat_item = _chat_items[i];
             var item_width = chat_item.get_width();
             var item_height = chat_item.get_height();
 
+            var origin = Graphene.Point() {
+                x = (chat_item.from_me ? (container_width - item_width - bubble_margin) : bubble_margin), 
+                y = y_offset 
+            };
+
+            var size = Graphene.Size() {
+                width = item_width,
+                height = item_height
+            };
+
+            var rect = Graphene.Rect() {
+                origin = origin,
+                size = size
+            };
+
+            // Skip drawing if this item is not in the viewport
+            if (viewport_rect.intersection(rect, null)) {
                 snapshot.save();
 
+                // Translate to the correct position
+                snapshot.translate(origin);
+
                 // Flip the y-axis, since our parent is upside down (so newest messages are at the bottom)
                 snapshot.scale(1.0f, -1.0f);
 
-            // Translate to the correct position
-            snapshot.translate(Graphene.Point() { 
-                x = (chat_item.from_me ? (container_width - item_width - bubble_margin) : bubble_margin), 
-                y = y_offset 
-            });
-
                 // Undo the y-axis flip, origin is top left 
                 snapshot.translate(Graphene.Point() { x = 0, y = -item_height });
 
                 chat_item.draw(snapshot);
                 snapshot.restore();
+            }
 
-            y_offset -= item_height + chat_item.vertical_padding;
+            y_offset += item_height + chat_item.vertical_padding;
         }
     }
 

src/transcript/transcript-view.vala

@@ -17,8 +17,10 @@ public class TranscriptView : Adw.Bin
             _model = value;
 
             if (value != null) {
-                // Reset scroll position
-                scrolled_window.vadjustment = new Gtk.Adjustment(0, 0, 0, 0, 0, 0);
+                // Reset scroll position by updating the existing adjustment
+                scrolled_window.vadjustment.value = 0;
+                scrolled_window.vadjustment.upper = 0;
+                scrolled_window.vadjustment.page_size = 0;
 
                 weak TranscriptView self = this;
                 messages_changed_handler_id = value.messages_changed.connect(() => {
@@ -56,8 +58,14 @@ public class TranscriptView : Adw.Bin
 
         scrolled_window.set_child(transcript_drawing_area);
         scrolled_window.add_css_class("message-list-scroller");
+        transcript_drawing_area.viewport = scrolled_window.vadjustment;
         container.set_content(scrolled_window);
 
+        // Connect to the adjustment's value_changed signal
+        scrolled_window.vadjustment.value_changed.connect(() => {
+            transcript_drawing_area.viewport = scrolled_window.vadjustment;
+        });
+
         var header_bar = new Adw.HeaderBar();
         title_label.single_line_mode = true;
         title_label.ellipsize = Pango.EllipsizeMode.END;