Change modal position to a fixed vertical distance from the top of th…

…e window (#4700)

### What

This avoids the modal moving vertically when content size changes.

**NOTE**: this PR would trigger the bug fixed in
emilk/egui#3721, so it needs an egui release
before being released. This is avoided by salting the modals' ID such as
to in invalidate existing caches.


https://github.com/rerun-io/rerun/assets/49431240/d2d472b5-7af2-4e95-9da1-a671fc80629f


### 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/4700/index.html)
* Using examples from latest `main` build:
[app.rerun.io](https://app.rerun.io/pr/4700/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/4700/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/4700)
- [Docs
preview](https://rerun.io/preview/e159b005a7a1d5f44e608013f00c1d479b7c341c/docs)
<!--DOCS-PREVIEW-->
- [Examples
preview](https://rerun.io/preview/e159b005a7a1d5f44e608013f00c1d479b7c341c/examples)
<!--EXAMPLES-PREVIEW-->
- [Recent benchmark results](https://build.rerun.io/graphs/crates.html)
- [Wasm size tracking](https://build.rerun.io/graphs/sizes.html)

crates/re_ui/src/modal.rs

@@ -1,3 +1,5 @@
+use egui::NumExt;
+
 /// Helper object to handle a [`Modal`] window.
 ///
 /// A [`Modal`] is typically held only so long as it is displayed, so it's typically stored in an
@@ -74,6 +76,25 @@ pub struct ModalResponse<R> {
 /// [`Modal`] fakes as a modal window, since egui [doesn't have them yet](https://github.com/emilk/egui/issues/686).
 /// This done by dimming the background and capturing clicks outside the window.
 ///
+/// The positioning of the modal is as follows:
+///
+/// ```text
+/// ┌─rerun window─────▲─────────────────────┐
+/// │                  │ 75px / 10%          │
+/// │          ╔═modal═▼══════════╗  ▲       │
+/// │          ║               ▲  ║  │       │
+/// │          ║ actual height │  ║  │       │
+/// │          ║      based on │  ║  │ max   │
+/// │          ║       content │  ║  │ height│
+/// │          ║               │  ║  │       │
+/// │          ║               ▼  ║  │       │
+/// │          ╚══════════════════╝  │       │
+/// │          │                  │  │       │
+/// │          └───────▲──────────┘  ▼       │
+/// │                  │ 75px / 10%          │
+/// └──────────────────▼─────────────────────┘
+/// ```
+///
 /// Note that [`Modal`] are typically used via the [`ModalHandler`] helper object to reduce boilerplate.
 pub struct Modal {
     title: String,
@@ -109,10 +130,18 @@ impl Modal {
 
         let mut open = ui.input(|i| !i.key_pressed(egui::Key::Escape));
 
+        let screen_height = ui.ctx().screen_rect().height();
+        let modal_vertical_margins = (75.0).at_most(screen_height * 0.1);
+
         let mut window = egui::Window::new(&self.title)
-            .pivot(egui::Align2::CENTER_CENTER)
-            .fixed_pos(ui.ctx().screen_rect().center())
+            //TODO(ab): workaround for https://github.com/emilk/egui/pull/3721 until we make a new egui release
+            .id(egui::Id::new(("modal", &self.title)))
+            .pivot(egui::Align2::CENTER_TOP)
+            .fixed_pos(
+                ui.ctx().screen_rect().center_top() + egui::vec2(0.0, modal_vertical_margins),
+            )
             .constrain_to(ui.ctx().screen_rect())
+            .max_height(screen_height - 2.0 * modal_vertical_margins)
             .collapsible(false)
             .resizable(true)
             .frame(egui::Frame {