Add support for exact width to PropertyContent (rerun-io#6325)

### What

This PR adds support for `PropertyContent::exact_width(true)`, which
makes it possible to create lists which only use the needed width (as
opposed to using `ui.available_width()`. This was previous available for
`LabelContent` and used (in the future) in the streams view.

`LabelContent` exploits the fact that the content is mainly a label, the
width of which can be computed with `egui`. For `PropertyContent` it's
tricker because we delegate the second column to a closure. Instead, we
track the max width in each `list_item_context` on frame n, and request
that width in frame n+1.

The major drawback of this approach is the potential flicker on the
first frame. Since the plan is to use it for tooltip, it will greatly
help to have emilk/egui#4471

### 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/6325?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/6325?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)!

- [PR Build Summary](https://build.rerun.io/pr/6325)
- [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/re_ui/src/list_item2/property_content.rs

@@ -1,6 +1,6 @@
 use egui::{text::TextWrapping, Align, Align2, NumExt as _, Ui};
 
-use super::{ContentContext, DesiredWidth, ListItemContent};
+use super::{ContentContext, DesiredWidth, LayoutInfoStack, ListItemContent};
 use crate::{Icon, ReUi};
 
 /// Closure to draw an icon left of the label.
@@ -20,6 +20,8 @@ struct PropertyActionButton<'a> {
 pub struct PropertyContent<'a> {
     label: egui::WidgetText,
     min_desired_width: f32,
+    exact_width: bool,
+
     icon_fn: Option<Box<IconFn<'a>>>,
     show_only_when_collapsed: bool,
     value_fn: Option<Box<PropertyValueFn<'a>>>,
@@ -37,6 +39,7 @@ impl<'a> PropertyContent<'a> {
         Self {
             label: label.into(),
             min_desired_width: 200.0,
+            exact_width: false,
             icon_fn: None,
             show_only_when_collapsed: true,
             value_fn: None,
@@ -54,6 +57,17 @@ impl<'a> PropertyContent<'a> {
         self
     }
 
+    /// Allocate the exact width required for the entire content.
+    ///
+    /// Note: this is done by tracking the maximum width in the current [`super::list_item_scope`]
+    /// during the previous frame, so this is effective on the second frame only. If the first frame
+    /// is actually rendered, this can lead to a flicker.
+    #[inline]
+    pub fn exact_width(mut self, exact_width: bool) -> Self {
+        self.exact_width = exact_width;
+        self
+    }
+
     /// Provide an [`Icon`] to be displayed on the left of the label.
     #[inline]
     pub fn with_icon(self, icon: &'a Icon) -> Self {
@@ -188,6 +202,7 @@ impl ListItemContent for PropertyContent<'_> {
             show_only_when_collapsed,
             value_fn,
             action_buttons,
+            exact_width: _,
         } = *self;
 
         // │                                                                              │
@@ -313,6 +328,11 @@ impl ListItemContent for PropertyContent<'_> {
                 let mut child_ui =
                     ui.child_ui(value_rect, egui::Layout::left_to_right(egui::Align::Center));
                 value_fn(re_ui, &mut child_ui, visuals);
+
+                context.layout_info.register_property_content_max_width(
+                    child_ui.ctx(),
+                    child_ui.min_rect().right() - context.layout_info.left_x,
+                );
             }
         }
 
@@ -336,7 +356,27 @@ impl ListItemContent for PropertyContent<'_> {
         }
     }
 
-    fn desired_width(&self, _re_ui: &ReUi, _ui: &Ui) -> DesiredWidth {
-        DesiredWidth::AtLeast(self.min_desired_width)
+    fn desired_width(&self, _re_ui: &ReUi, ui: &Ui) -> DesiredWidth {
+        let layout_info = LayoutInfoStack::top(ui.ctx());
+        if self.exact_width {
+            if let Some(max_width) = layout_info.property_content_max_width {
+                let mut desired_width = max_width + layout_info.left_x - ui.max_rect().left();
+
+                // TODO(ab): ideally there wouldn't be as much code duplication with `Self::ui`
+                let action_button_dimension =
+                    ReUi::small_icon_size().x + 2.0 * ui.spacing().button_padding.x;
+                let reserve_action_button_space =
+                    self.action_buttons.is_some() || layout_info.reserve_action_button_space;
+                if reserve_action_button_space {
+                    desired_width += action_button_dimension + ReUi::text_to_icon_padding();
+                }
+
+                DesiredWidth::Exact(desired_width.ceil())
+            } else {
+                DesiredWidth::AtLeast(self.min_desired_width)
+            }
+        } else {
+            DesiredWidth::AtLeast(self.min_desired_width)
+        }
     }
 }

crates/re_ui/src/list_item2/scope.rs

@@ -51,6 +51,11 @@ struct LayoutStatistics {
     ///
     /// The width is calculated from [`LayoutInfo::left_x`] to the right edge of the item.
     max_item_width: Option<f32>,
+
+    /// `PropertyContent` only — max content width in the current scope.
+    ///
+    /// This value is measured from `left_x`.
+    property_content_max_width: Option<f32>,
 }
 
 impl LayoutStatistics {
@@ -116,6 +121,11 @@ pub struct LayoutInfo {
 
     /// Scope id, used to retrieve the corresponding [`LayoutStatistics`].
     scope_id: egui::Id,
+
+    /// `PropertyContent` only — last frame's max content width, to be used in `desired_width()`
+    ///
+    /// This value is measured from `left_x`.
+    pub(crate) property_content_max_width: Option<f32>,
 }
 
 impl Default for LayoutInfo {
@@ -125,6 +135,7 @@ impl Default for LayoutInfo {
             left_column_width: None,
             reserve_action_button_space: true,
             scope_id: egui::Id::NULL,
+            property_content_max_width: None,
         }
     }
 }
@@ -158,6 +169,16 @@ impl LayoutInfo {
             stats.max_item_width = stats.max_item_width.map(|v| v.max(width)).or(Some(width));
         });
     }
+
+    /// `PropertyContent` only — register max content width in the current scope
+    pub(super) fn register_property_content_max_width(&self, ctx: &egui::Context, width: f32) {
+        LayoutStatistics::update(ctx, self.scope_id, |stats| {
+            stats.property_content_max_width = stats
+                .property_content_max_width
+                .map(|v| v.max(width))
+                .or(Some(width));
+        });
+    }
 }
 
 /// Stack of [`LayoutInfo`]s.
@@ -255,6 +276,7 @@ pub fn list_item_scope<R>(
         left_column_width,
         reserve_action_button_space: layout_stats.is_action_button_used,
         scope_id,
+        property_content_max_width: layout_stats.property_content_max_width,
     };
 
     // push, run, pop