Clamp time panel height to avoid visual glitches (#3169)

### What

Fixes #3152

*Note to reviewer*: I had to resort to some semi-hack to measure the
height of "everything above the viewport", i.e. windows title bar vs.
heading banner on web (which are significantly different). That's the
`screen_header_height` variable. Open to suggestion to make that
cleaner.

### 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 [demo.rerun.io](https://demo.rerun.io/pr/3169) (if
applicable)~~ PR demo somehow failed, here is the [demo for commit
8fb79bf](https://demo.rerun.io/commit/8fb79bf/examples/structure_from_motion/)

- [PR Build Summary](https://build.rerun.io/pr/3169)
- [Docs
preview](https://rerun.io/preview/d110bc598c8fd1644d1ad658b45ece7d9c752df0/docs)
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crates/re_time_panel/src/lib.rs

@@ -64,6 +64,10 @@ impl TimePanel {
         ui: &mut egui::Ui,
         time_panel_expanded: bool,
     ) {
+        // this is the size of everything above the central panel (window title bar, top bar on web,
+        // etc.)
+        let screen_header_height = ui.cursor().top();
+
         let top_bar_height = 28.0;
         let margin = ctx.re_ui.bottom_panel_margin();
         let mut panel_frame = ctx.re_ui.bottom_panel_frame();
@@ -76,7 +80,7 @@ impl TimePanel {
             panel_frame.inner_margin.right = 0.0;
         }
 
-        let screen_height = ui.ctx().screen_rect().width();
+        let window_height = ui.ctx().screen_rect().height();
 
         let collapsed = egui::TopBottomPanel::bottom("time_panel_collapsed")
             .resizable(false)
@@ -85,12 +89,14 @@ impl TimePanel {
             .default_height(44.0);
 
         let min_height = 150.0;
+        let min_top_space = 150.0 + screen_header_height;
         let expanded = egui::TopBottomPanel::bottom("time_panel_expanded")
             .resizable(true)
             .show_separator_line(false)
             .frame(panel_frame)
             .min_height(min_height)
-            .default_height((0.25 * screen_height).clamp(min_height, 250.0).round());
+            .max_height((window_height - min_top_space).at_least(min_height).round())
+            .default_height((0.25 * window_height).clamp(min_height, 250.0).round());
 
         egui::TopBottomPanel::show_animated_between_inside(
             ui,