Bug fix: always show latest data in follow-mode (#7425)

### What
* Found while testing #7422

Always show the latest data in follow-mode, by advancing the time at the
start of the frame rather than at the end of it.

### Checklist
* [x] I have read and agree to [Contributor
Guide](https://github.com/rerun-io/rerun/blob/main/CONTRIBUTING.md) and
the [Code of
Conduct](https://github.com/rerun-io/rerun/blob/main/CODE_OF_CONDUCT.md)
* [x] I've included a screenshot or gif (if applicable)
* [x] I have tested the web demo (if applicable):
* Using examples from latest `main` build:
[rerun.io/viewer](https://rerun.io/viewer/pr/7425?manifest_url=https://app.rerun.io/version/main/examples_manifest.json)
* Using full set of examples from `nightly` build:
[rerun.io/viewer](https://rerun.io/viewer/pr/7425?manifest_url=https://app.rerun.io/version/nightly/examples_manifest.json)
* [x] The PR title and labels are set such as to maximize their
usefulness for the next release's CHANGELOG
* [x] If applicable, add a new check to the [release
checklist](https://github.com/rerun-io/rerun/blob/main/tests/python/release_checklist)!
* [x] If have noted any breaking changes to the log API in
`CHANGELOG.md` and the migration guide

- [PR Build Summary](https://build.rerun.io/pr/7425)
- [Recent benchmark results](https://build.rerun.io/graphs/crates.html)
- [Wasm size tracking](https://build.rerun.io/graphs/sizes.html)

To run all checks from `main`, comment on the PR with `@rerun-bot
full-check`.

crates/viewer/re_viewer/src/app_state.rs

@@ -272,6 +272,10 @@ impl AppState {
             focused_item,
         };
 
+        // We move the time at the very start of the frame,
+        // so that we always show the latest data when we're in "follow" mode.
+        move_time(&ctx, recording, rx);
+
         // Update the viewport. May spawn new views and handle queued requests (like screenshots).
         viewport.on_frame_start(&ctx);
 
@@ -466,41 +470,6 @@ impl AppState {
         // Process deferred layout operations and apply updates back to blueprint
         viewport.update_and_sync_tile_tree_to_blueprint(&ctx);
 
-        {
-            // We move the time at the very end of the frame,
-            // so we have one frame to see the first data before we move the time.
-            let dt = ui.ctx().input(|i| i.stable_dt);
-
-            // Are we still connected to the data source for the current store?
-            let more_data_is_coming = if let Some(store_source) = &recording.data_source {
-                rx.sources().iter().any(|s| s.as_ref() == store_source)
-            } else {
-                false
-            };
-
-            let recording_needs_repaint = ctx.rec_cfg.time_ctrl.write().update(
-                recording.times_per_timeline(),
-                dt,
-                more_data_is_coming,
-            );
-
-            let blueprint_needs_repaint = if ctx.app_options.inspect_blueprint_timeline {
-                ctx.blueprint_cfg.time_ctrl.write().update(
-                    ctx.store_context.blueprint.times_per_timeline(),
-                    dt,
-                    more_data_is_coming,
-                )
-            } else {
-                re_viewer_context::NeedsRepaint::No
-            };
-
-            if recording_needs_repaint == re_viewer_context::NeedsRepaint::Yes
-                || blueprint_needs_repaint == re_viewer_context::NeedsRepaint::Yes
-            {
-                ui.ctx().request_repaint();
-            }
-        }
-
         if WATERMARK {
             ui.ctx().paint_watermark();
         }
@@ -537,6 +506,39 @@ impl AppState {
     }
 }
 
+fn move_time(ctx: &ViewerContext<'_>, recording: &EntityDb, rx: &ReceiveSet<LogMsg>) {
+    let dt = ctx.egui_ctx.input(|i| i.stable_dt);
+
+    // Are we still connected to the data source for the current store?
+    let more_data_is_coming = if let Some(store_source) = &recording.data_source {
+        rx.sources().iter().any(|s| s.as_ref() == store_source)
+    } else {
+        false
+    };
+
+    let recording_needs_repaint = ctx.rec_cfg.time_ctrl.write().update(
+        recording.times_per_timeline(),
+        dt,
+        more_data_is_coming,
+    );
+
+    let blueprint_needs_repaint = if ctx.app_options.inspect_blueprint_timeline {
+        ctx.blueprint_cfg.time_ctrl.write().update(
+            ctx.store_context.blueprint.times_per_timeline(),
+            dt,
+            more_data_is_coming,
+        )
+    } else {
+        re_viewer_context::NeedsRepaint::No
+    };
+
+    if recording_needs_repaint == re_viewer_context::NeedsRepaint::Yes
+        || blueprint_needs_repaint == re_viewer_context::NeedsRepaint::Yes
+    {
+        ctx.egui_ctx.request_repaint();
+    }
+}
+
 fn recording_config_entry<'cfgs>(
     configs: &'cfgs mut HashMap<StoreId, RecordingConfig>,
     id: StoreId,