surface: selection region keeps up with selection scrolling

src/Surface.zig

@@ -3497,7 +3497,6 @@ pub fn cursorPosCallback(
         // Only send a message when outside the viewport and
         // selection scrolling is not currently active.
         if ((pos.y <= 1 or pos.y > max_y - 1) and !self.io_thread.scroll_active) {
-            // TODO: the selection region ideally should keep up with this
             self.io.queueMessage(.{ .selection_scroll = true }, .locked);
         }
 
@@ -3593,7 +3592,7 @@ fn dragLeftClickTriple(
     try self.setSelection(sel);
 }
 
-fn dragLeftClickSingle(
+pub fn dragLeftClickSingle(
     self: *Surface,
     drag_pin: terminal.Pin,
     xpos: f64,

src/termio/Thread.zig

@@ -470,10 +470,27 @@ fn selectionScrollCallback(
     const cb = cb_ orelse return .disarm;
     const surface = cb.io.surface_mailbox.surface;
     const pos = try surface.rt_surface.getCursorPos();
+    const pos_vp = surface.posToViewport(pos.x, pos.y);
+    const screen = &cb.io.renderer_state.terminal.screen;
     const delta: isize = if (pos.y < 0) -1 else 1;
 
     try cb.io.terminal.scrollViewport(.{ .delta = delta });
 
+    // Always the case, but doesn't hurt to check
+    if (surface.mouse.left_click_count == 1) {
+        const pin = screen.pages.pin(.{
+            .viewport = .{
+                .x = pos_vp.x,
+                .y = pos_vp.y,
+            },
+        }) orelse {
+            if (comptime std.debug.runtime_safety) unreachable;
+            return .rearm;
+        };
+
+        surface.dragLeftClickSingle(pin, pos.x) catch {};
+    }
+
     // Notify the renderer that it should repaint immediately after scrolling
     cb.io.renderer_wakeup.notify() catch {};