Persist Auto vs Columns for Grid into the Blueprint (#4686)

### What
 - Resolves: #4685

### 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 newly built examples:
[app.rerun.io](https://app.rerun.io/pr/4686/index.html)
* Using examples from latest `main` build:
[app.rerun.io](https://app.rerun.io/pr/4686/index.html?manifest_url=https://app.rerun.io/version/main/examples_manifest.json)
* Using full set of examples from `nightly` build:
[app.rerun.io](https://app.rerun.io/pr/4686/index.html?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

- [PR Build Summary](https://build.rerun.io/pr/4686)
- [Docs
preview](https://rerun.io/preview/eeab9f0740e014ab7be07368b97a842eabf0359e/docs)
<!--DOCS-PREVIEW-->
- [Examples
preview](https://rerun.io/preview/eeab9f0740e014ab7be07368b97a842eabf0359e/examples)
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- [Recent benchmark results](https://build.rerun.io/graphs/crates.html)
- [Wasm size tracking](https://build.rerun.io/graphs/sizes.html)

crates/re_types/definitions/rerun/blueprint.fbs

@@ -4,6 +4,7 @@ include "./blueprint/components/auto_space_views.fbs";
 include "./blueprint/components/container_kind.fbs";
 include "./blueprint/components/entities_determined_by_user.fbs";
 include "./blueprint/components/entity_properties_component.fbs";
+include "./blueprint/components/grid_columns.fbs";
 include "./blueprint/components/included_contents.fbs";
 include "./blueprint/components/included_queries.fbs";
 include "./blueprint/components/included_space_views.fbs";

crates/re_types/definitions/rerun/blueprint/archetypes/container_blueprint.fbs

@@ -32,7 +32,9 @@ table ContainerBlueprint (
     /// For `Horizontal`/`Vertical` containers, the length of this list should always match the number of contents.
     primary_weights: rerun.blueprint.components.PrimaryWeights ("attr.rerun.component_optional", nullable, order: 400);
 
-    /// The weights of the secondary axis. For `Grid` this is the row weights. Ignored for `Horizontal`/`Vertical` containers.
+    /// The weights of the secondary axis. For `Grid` this is the row weights.
+    ///
+    /// Ignored for `Horizontal`/`Vertical` containers.
     secondary_weights: rerun.blueprint.components.SecondaryWeights ("attr.rerun.component_optional", nullable, order: 500);
 
     /// Which tab is active.
@@ -44,4 +46,11 @@ table ContainerBlueprint (
     ///
     /// Defaults to true if not specified.
     visible: rerun.blueprint.components.Visible ("attr.rerun.component_optional", nullable, order: 700);
+
+    /// How many columns this grid should have.
+    ///
+    /// If unset, the grid layout will be auto.
+    ///
+    /// Ignored for `Horizontal`/`Vertical` containers.
+    grid_columns: rerun.blueprint.components.GridColumns ("attr.rerun.component_optional", nullable, order: 800);
 }

crates/re_types/definitions/rerun/blueprint/components/grid_columns.fbs

@@ -0,0 +1,20 @@
+include "arrow/attributes.fbs";
+include "python/attributes.fbs";
+include "rust/attributes.fbs";
+
+include "rerun/datatypes.fbs";
+include "rerun/attributes.fbs";
+
+namespace rerun.blueprint.components;
+
+
+/// How many columns a grid container should have.
+table GridColumns (
+    "attr.rerun.scope": "blueprint",
+    "attr.docs.unreleased",
+    "attr.rust.derive": "Default, PartialEq, Eq, PartialOrd, Ord",
+    "attr.rust.override_crate": "re_viewport"
+) {
+    /// The number of columns.
+    columns: uint (order: 100);
+}

crates/re_viewer/src/ui/selection_panel.rs

@@ -536,6 +536,8 @@ fn container_top_level_properties(
                         }
                     }
 
+                    let original_layout = grid.layout;
+
                     egui::ComboBox::from_id_source("container_grid_columns")
                         .selected_text(grid_layout_to_string(&grid.layout))
                         .show_ui(ui, |ui| {
@@ -559,6 +561,10 @@ fn container_top_level_properties(
                             }
                         });
 
+                    // TODO(jleibs): Manually marking edited like this is way too error prone.
+                    // Need to detect this in a better way.
+                    viewport.edited |= original_layout != grid.layout;
+
                     ui.end_row();
                 }