Add support for drag-and-drop in blueprint tree (#4910)

### What

This PR adds support for drag-and-drop in the blueprint tree.

- Fixes #2652


https://github.com/rerun-io/rerun/assets/49431240/4d5f47ae-6d79-4ad0-a05d-b7eaaac18659


It has the following known limitations/future work:
- Only a single item may be dragged at a time. If multiple items are
selected when a drag starts, the selection is set to contain only the
dragged item.
  - #4887
- Only space view and containers may be dragged. The drag and drop
operations only operate on the view hierarchy (not the space view data).
- The root container has a special behaviour: it cannot be successfully
dragged, and nothing can be dragged before/after it. This is consistent
with it being the root container, but inconsistent with how the UI is
presented.
  - #4909
- Because of how Grid container work (they support "holes", and tend to
swap instead of reflow items when reordering), some drag operation have
additional order side effects within the destination (grid) container.
- #4916

### 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/4910/index.html)
* Using examples from latest `main` build:
[app.rerun.io](https://app.rerun.io/pr/4910/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/4910/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/4910)
- [Docs
preview](https://rerun.io/preview/b4084fe32cd0466fe6acd4807a33545091c9e1ba/docs)
<!--DOCS-PREVIEW-->
- [Examples
preview](https://rerun.io/preview/b4084fe32cd0466fe6acd4807a33545091c9e1ba/examples)
<!--EXAMPLES-PREVIEW-->
- [Recent benchmark results](https://build.rerun.io/graphs/crates.html)
- [Wasm size tracking](https://build.rerun.io/graphs/sizes.html)

---------

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

Cargo.toml

@@ -294,6 +294,6 @@ emath = { git = "https://github.com/emilk/egui.git", rev = "ab39420c2933d2e40299
 # egui-wgpu = { path = "../../egui/crates/egui-wgpu" }
 # emath = { path = "../../egui/crates/emath" }
 
-# egui_tiles = { git = "https://github.com/rerun-io/egui_tiles", rev = "b6e4fd457b2eee2c671747ead12f4a20feb380e8" } # Merge of: https://github.com/rerun-io/egui_tiles/pull/41
+egui_tiles = { git = "https://github.com/rerun-io/egui_tiles", rev = "35e711283e7a021ca425d9fbd8e7581548971f49" } # master 2024-01-26
 
 # egui_commonmark = { git = "https://github.com/rerun-io/egui_commonmark", rev = "3d83a92f995a1d18ab1172d0b129d496e0eedaae" } # Update to egui 0.25 https://github.com/lampsitter/egui_commonmark/pull/27

crates/re_ui/src/drag_and_drop.rs

@@ -0,0 +1,320 @@
+//! Helpers for drag and drop support for reordering hierarchical lists.
+//!
+//! Works well in combination with [`crate::list_item::ListItem`].
+
+/// Context information about the hovered item.
+///
+/// This is used by [`find_drop_target`] to compute the [`DropTarget`], if any.
+pub struct ItemContext<ItemId: Copy> {
+    /// ID of the item being hovered during drag
+    pub id: ItemId,
+
+    /// Can this item "contain" the currently dragged item?
+    pub is_container: bool,
+
+    /// ID of the parent of this item.
+    pub parent_id: ItemId,
+
+    /// Position of this item within its parent.
+    pub position_index_in_parent: usize,
+
+    /// ID of the container just before this item within the parent, if such a container exists.
+    pub previous_container_id: Option<ItemId>,
+}
+
+/// Drop target information, including where to draw the drop indicator and where to insert the dragged item.
+#[derive(Clone, Debug)]
+pub struct DropTarget<ItemId: Copy> {
+    /// Range of X coordinates for the drag target indicator
+    pub indicator_span_x: egui::Rangef,
+
+    /// Y coordinate for drag target indicator
+    pub indicator_position_y: f32,
+
+    /// Destination container ID
+    pub target_parent_id: ItemId,
+
+    /// Destination position within the container
+    pub target_position_index: usize,
+}
+
+impl<ItemId: Copy> DropTarget<ItemId> {
+    pub fn new(
+        indicator_span_x: egui::Rangef,
+        indicator_position_y: f32,
+        target_parent_id: ItemId,
+        target_position_index: usize,
+    ) -> Self {
+        Self {
+            indicator_span_x,
+            indicator_position_y,
+            target_parent_id,
+            target_position_index,
+        }
+    }
+}
+
+/// Compute the geometry of the drag cursor and where the dragged item should be inserted.
+///
+/// This function performs the following tasks:
+/// - based on `item_rect` and `body_rect`, establish the geometry of actual drop zones (see below)
+/// - test the mouse cursor against these zones
+/// - if one is a match:
+///   - compute the geometry of a drop insertion indicator
+///   - use the context provided in `item_context` to return the "logical" drop target (ie. the target container and
+///     position within it)
+///
+/// This function implements the following logic:
+/// ```text
+///
+///                        insert         insert last in container before me
+///                      before me           (if any) or insert before me
+///                          │                             │
+///                      ╔═══▼═════════════════════════════▼══════════════════╗
+///                      ║      │                                             ║
+///         leaf item    ║ ─────┴──────────────────────────────────────────── ║
+///                      ║                                                    ║
+///                      ╚═════════════════════▲══════════════════════════════╝
+///                                            │
+///                                     insert after me
+///
+///
+///                        insert         insert last in container before me
+///                      before me           (if any) or insert before me
+///                          │                             │
+///                      ╔═══▼═════════════════════════════▼══════════════════╗
+///         leaf item    ║      │                                             ║
+///         with body    ║ ─────┴──────────────────────────────────────────── ║
+///                      ║                                                    ║
+///                      ╚══════╦══════════════════════════════════════▲══════╣ ─┐
+///                      │      ║                                      │      ║  │
+///                      │      ║                                   insert    ║  │
+///                      │      ║                                  after me   ║  │
+///                      │      ╠══                                         ══╣  │
+///                      │      ║             no insertion possible           ║  │
+///                      │      ║             here by definition of           ║  │ body
+///                      │      ║              parent being a leaf            ║  │
+///                      │      ╠══                                         ══╣  │
+///                      │      ║                                             ║  │
+///                      │      ║                                             ║  │
+///                      │      ║                                             ║  │
+///                      └──▲── ╚══════════════════════════▲══════════════════╝ ─┘
+///                         │                              │
+///                      insert                         insert
+///                     after me                       after me
+///
+///
+///                        insert         insert last in container before me
+///                      before me           (if any) or insert before me
+///                          │                             │
+///                      ╔═══▼═════════════════════════════▼══════════════════╗
+///    container item    ║      │                                             ║
+///  (empty/collapsed    ║ ─────┼──────────────────────────────────────────── ║
+///             body)    ║      │                                             ║
+///                      ╚═══▲═════════════════════════════▲══════════════════╝
+///                          │                             │
+///                       insert                   insert inside me
+///                      after me                     at pos = 0
+///
+///
+///                        insert         insert last in container before me
+///                      before me           (if any) or insert before me
+///                          │                             │
+///                      ╔═══▼═════════════════════════════▼══════════════════╗
+///    container item    ║      │                                             ║
+///         with body    ║ ─────┴──────────────────────────────────────────── ║
+///                      ║                                                    ║
+///                      ╚═▲════╦═════════════════════════════════════════════╣ ─┐
+///                        │    ║                                             ║  │
+///                     insert  ║                                             ║  │
+///                  inside me  ║                                             ║  │
+///                 at pos = 0  ╠══                                         ══╣  │
+///                             ║                  same logic                 ║  │
+///                             ║                 recursively                 ║  │ body
+///                     insert  ║                 applied here                ║  │
+///                   after me  ╠══                                         ══╣  │
+///                        │    ║                                             ║  │
+///                      ┌─▼─── ║                                             ║  │
+///                      │      ║                                             ║  │
+///                      └───── ╚═════════════════════════════════════════════╝ ─┘
+/// ```
+///
+/// Here are a few observations of the above that help navigate the "if-statement-of-death"
+/// in the implementation:
+/// - The top parts of the item are treated the same in all four cases.
+/// - Handling of the body can be simplified by making the sensitive area either a small
+///   corner (container case), or the entire body (leaf case). Then, that area always maps
+///   to "insert after me".
+/// - The bottom parts have the most difference between cases and need case-by-case handling.
+///   In both leaf item cases, the entire bottom part maps to "insert after me", though.
+///
+/// **Note**: in debug builds, press `Alt` to visualize the drag zones while dragging.
+pub fn find_drop_target<ItemId: Copy>(
+    ui: &egui::Ui,
+    item_context: &ItemContext<ItemId>,
+    item_rect: egui::Rect,
+    body_rect: Option<egui::Rect>,
+    item_height: f32,
+) -> Option<DropTarget<ItemId>> {
+    let indent = ui.spacing().indent;
+    let item_id = item_context.id;
+    let is_container = item_context.is_container;
+    let parent_id = item_context.parent_id;
+    let pos_in_parent = item_context.position_index_in_parent;
+
+    // For both leaf and containers we have two drag zones on the upper half of the item.
+    let (top, mut bottom) = item_rect.split_top_bottom_at_fraction(0.5);
+    let (left_top, top) = top.split_left_right_at_x(top.left() + indent);
+
+    // For the lower part of the item, the story is more complicated:
+    // - for leaf item, we have a single drag zone on the entire lower half
+    // - for container item, we must distinguish between the indent part and the rest, plus check some area in the
+    //   body
+    let mut left_bottom = egui::Rect::NOTHING;
+    if is_container {
+        (left_bottom, bottom) = bottom.split_left_right_at_x(bottom.left() + indent);
+    }
+
+    // For the body area we have two cases:
+    // - container item: it's handled recursively by the nested items, so we only need to check a small area down
+    //   left, which maps to "insert after me"
+    // - leaf item: the entire body area, if any, cannot receive a drag (by definition) and thus homogeneously maps
+    //   to "insert after me"
+    let body_insert_after_me_area = if let Some(body_rect) = body_rect {
+        if item_context.is_container {
+            egui::Rect::from_two_pos(
+                body_rect.left_bottom() + egui::vec2(indent, -item_height / 2.0),
+                body_rect.left_bottom(),
+            )
+        } else {
+            body_rect
+        }
+    } else {
+        egui::Rect::NOTHING
+    };
+
+    // body rect, if any AND it actually contains something
+    let non_empty_body_rect = body_rect.filter(|r| r.height() > 0.0);
+
+    // visualize the drag zones in debug builds, when the `Alt` key is pressed during drag
+    #[cfg(debug_assertions)]
+    {
+        // Visualize the drag zones
+        if ui.input(|i| i.modifiers.alt) {
+            ui.ctx()
+                .debug_painter()
+                .debug_rect(top, egui::Color32::RED, "t");
+            ui.ctx()
+                .debug_painter()
+                .debug_rect(bottom, egui::Color32::GREEN, "b");
+
+            ui.ctx().debug_painter().debug_rect(
+                left_top,
+                egui::Color32::RED.gamma_multiply(0.5),
+                "lt",
+            );
+            ui.ctx().debug_painter().debug_rect(
+                left_bottom,
+                egui::Color32::GREEN.gamma_multiply(0.5),
+                "lb",
+            );
+            ui.ctx().debug_painter().debug_rect(
+                body_insert_after_me_area,
+                egui::Color32::YELLOW,
+                "bdy",
+            );
+        }
+    }
+
+    /* ===== TOP SECTIONS (same leaf/container items) ==== */
+    if ui.rect_contains_pointer(left_top) {
+        // insert before me
+        Some(DropTarget::new(
+            item_rect.x_range(),
+            top.top(),
+            parent_id,
+            pos_in_parent,
+        ))
+    } else if ui.rect_contains_pointer(top) {
+        // insert last in the previous container if any, else insert before me
+        if let Some(previous_container_id) = item_context.previous_container_id {
+            Some(DropTarget::new(
+                (item_rect.left() + indent..=item_rect.right()).into(),
+                top.top(),
+                previous_container_id,
+                usize::MAX,
+            ))
+        } else {
+            Some(DropTarget::new(
+                item_rect.x_range(),
+                top.top(),
+                parent_id,
+                pos_in_parent,
+            ))
+        }
+    }
+    /* ==== BODY SENSE AREA ==== */
+    else if ui.rect_contains_pointer(body_insert_after_me_area) {
+        // insert after me in my parent
+        Some(DropTarget::new(
+            item_rect.x_range(),
+            body_insert_after_me_area.bottom(),
+            parent_id,
+            pos_in_parent + 1,
+        ))
+    }
+    /* ==== BOTTOM SECTIONS (leaf item) ==== */
+    else if !is_container {
+        if ui.rect_contains_pointer(bottom) {
+            let position_y = if let Some(body_rect) = non_empty_body_rect {
+                body_rect.bottom()
+            } else {
+                bottom.bottom()
+            };
+
+            // insert after me
+            Some(DropTarget::new(
+                item_rect.x_range(),
+                position_y,
+                parent_id,
+                pos_in_parent + 1,
+            ))
+        } else {
+            None
+        }
+    }
+    /* ==== BOTTOM SECTIONS (container item) ==== */
+    else if let Some(body_rect) = non_empty_body_rect {
+        if ui.rect_contains_pointer(left_bottom) || ui.rect_contains_pointer(bottom) {
+            // insert at pos = 0 inside me
+            Some(DropTarget::new(
+                (body_rect.left() + indent..=body_rect.right()).into(),
+                left_bottom.bottom(),
+                item_id,
+                0,
+            ))
+        } else {
+            None
+        }
+    } else if ui.rect_contains_pointer(left_bottom) {
+        // insert after me in my parent
+        Some(DropTarget::new(
+            item_rect.x_range(),
+            left_bottom.bottom(),
+            parent_id,
+            pos_in_parent + 1,
+        ))
+    } else if ui.rect_contains_pointer(bottom) {
+        // insert at pos = 0 inside me
+        Some(DropTarget::new(
+            (item_rect.left() + indent..=item_rect.right()).into(),
+            bottom.bottom(),
+            item_id,
+            0,
+        ))
+    }
+    /* ==== Who knows where else the mouse cursor might wander… ¯\_(ツ)_/¯ ==== */
+    else {
+        None
+    }
+}

crates/re_ui/src/lib.rs

@@ -3,6 +3,7 @@
 mod command;
 mod command_palette;
 mod design_tokens;
+pub mod drag_and_drop;
 pub mod egui_helpers;
 pub mod icons;
 mod layout_job_builder;