Render content-only when a task reads more lines into a view

Reading more lines into a lazy-loaded view (e.g. a diff being scrolled)
never changes the window layout, and after the first screenful it
doesn't even change the visible content - the new lines land below the
viewport, so the only thing that changes on screen is the scrollbar
thumb. Yet each read triggered a full render: a layout pass plus a
redraw of every view. On a slow terminal that full-screen repaint on
every read is a big part of why scrolling through a not-yet-fully-read
diff stutters.

Route the task's refresh through a content-only render instead. It
skips the layout pass and only redraws the views whose content changed,
leaving tcell's cell-level dirty tracking to emit just the cells that
actually differ (in the steady state, the scrollbar column).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

pkg/gui/tasks_adapter.go

@@ -118,7 +118,12 @@ func (gui *Gui) getManager(view *gocui.View) *tasks.ViewBufferManager {
 				view.Reset()
 			},
 			func() {
-				gui.render()
+				// As the task reads more lines, the only thing that changes is the
+				// view's content (and its scrollbar); the window layout doesn't. So a
+				// content-only render is enough, and it's much cheaper than a full
+				// layout-and-redraw on every read - which matters a lot when reading
+				// a long diff, where reads happen repeatedly as the user scrolls.
+				gui.renderContentOnly()
 			},
 			func() {
 				// Need to check if the content of the view is well past the origin.

pkg/gui/view_helpers.go

@@ -121,6 +121,14 @@ func (gui *Gui) render() {
 	gui.c.OnUIThread(func() error { return nil })
 }
 
+// renderContentOnly triggers a re-render that skips the layout pass and only
+// redraws the views whose content changed (relying on tcell's cell-level dirty
+// tracking to emit just the cells that actually differ). Use it when only a
+// view's content changed, not the window layout.
+func (gui *Gui) renderContentOnly() {
+	gui.c.OnUIThreadContentOnly(func() error { return nil })
+}
+
 // postRefreshUpdate is to be called on a context after the state that it depends on has been refreshed
 // if the context's view is set to another context we do nothing.
 // if the context's view is the current view we trigger a focus; re-selecting the current item.