Fix where Panel puts its separator line, and how much room it reserves (#8382)

Two fixes to the separator line of `Panel` (`resolve_frame` was added in
#8367):

* **Reserve room only when the line is always drawn.** Before,
`show_separator_line || resizable` reserved the line's thickness, so a
resizable panel that opted out still got a permanently visible gap along
its inner edge — space held for a line only drawn transiently, while
hovering or dragging the resize handle.
* **Paint the line outside the frame's outline**, in room reserved in
`Frame::outer_margin` rather than `inner_margin`, so going outwards from
the panel contents you get:

  `contents | inner_margin | stroke | separator line | outer_margin`

Previously the line landed on top of the frame's outline (or outside its
outer margin). Default panels — no stroke, no outer margin — are
unchanged pixel-wise.

Found in the Rerun viewer: the time panel is
`.resizable(true).show_separator_line(false)` and draws its own top
line, so the extra 1pt landed above the top bar's buttons, making them
look 1pt too low.

Tests in `tests/egui_tests/tests/test_panel_separator_line.rs`, both
spanning `show_separator_line` on/off × resize handle hovered/not:
snapshots of a top panel with a garish outline, plus a pixel probe
across the inner edge of a panel on each of the four sides. Both fail on
`main`.

* [x] I have followed the instructions in the PR template

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

tests/egui_tests/tests/snapshots/panel_separator_line/separator_off_hovered.png

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tests/egui_tests/tests/snapshots/panel_separator_line/separator_off_idle.png

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tests/egui_tests/tests/snapshots/panel_separator_line/separator_on_hovered.png

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tests/egui_tests/tests/snapshots/panel_separator_line/separator_on_idle.png

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tests/egui_tests/tests/test_panel_separator_line.rs

@@ -0,0 +1,105 @@
+//! Snapshot tests for where a [`Panel`] puts its separator line, and how much room it reserves.
+//!
+//! Going outwards from the panel contents, the order is:
+//!
+//! contents | `Frame::inner_margin` | `Frame::stroke` | separator line | `Frame::outer_margin`
+//!
+//! i.e. the line is painted _outside_ the frame's outline, in room the panel reserves for it in the
+//! frame's outer margin. A panel that opted out of the separator line must not reserve that room,
+//! or it ends up with a permanently visible gap along that edge — even though it is `resizable` and
+//! therefore still shows a line while hovered or dragged.
+//!
+//! The snapshots span `show_separator_line` on/off × resize handle hovered/not. The panel uses a
+//! garish frame outline and separator colors so both are unmistakable, and its only content is a
+//! [`egui::SelectableLabel`] vertically centered in the panel: if the panel reserves room it
+//! shouldn't, the label drifts off center.
+
+use egui::{Color32, CornerRadius, Frame, Margin, Panel, Pos2, Stroke, Vec2};
+use egui_kittest::{Harness, SnapshotResults};
+
+/// [`Frame::fill`] of the test panel.
+const FILL: Color32 = Color32::from_rgb(20, 20, 40);
+
+/// [`Frame::stroke`] color of the test panel.
+const OUTLINE: Color32 = Color32::from_rgb(255, 0, 255);
+
+/// The dim, always-visible separator line (`noninteractive.bg_stroke`).
+const SEPARATOR: Color32 = Color32::from_rgb(0, 255, 0);
+
+/// The bright separator line shown while the resize handle is hovered (`hovered.fg_stroke`).
+const HOVERED_SEPARATOR: Color32 = Color32::from_rgb(255, 255, 0);
+
+const PANEL_ID: &str = "test_panel";
+
+fn build_harness(show_separator_line: bool) -> Harness<'static> {
+    let mut harness = Harness::builder()
+        .with_size(Vec2::new(200.0, 120.0))
+        // So the thin lines are legible to a human reviewing the snapshots:
+        .with_pixels_per_point(2.0)
+        .build_ui(move |ui| {
+            // Loud, distinguishable colors, so we can tell the separator line, the frame outline
+            // and the frame fill apart.
+            let widgets = &mut ui.visuals_mut().widgets;
+            widgets.noninteractive.bg_stroke = Stroke::new(1.0, SEPARATOR);
+            widgets.hovered.fg_stroke = Stroke::new(1.0, HOVERED_SEPARATOR);
+
+            let frame = Frame::new()
+                .fill(FILL)
+                .stroke(Stroke::new(2.0, OUTLINE))
+                .corner_radius(CornerRadius::ZERO)
+                .inner_margin(Margin::same(4))
+                .outer_margin(Margin::same(2));
+
+            Panel::top(PANEL_ID)
+                .frame(frame)
+                .resizable(true)
+                .default_size(60.0)
+                .show_separator_line(show_separator_line)
+                .show(ui, |ui| {
+                    // Vertically centered in whatever room the panel gave us.
+                    ui.horizontal_centered(|ui| {
+                        let _ = ui.selectable_label(true, "Centered");
+                    });
+                });
+
+            egui::CentralPanel::default()
+                .frame(Frame::default().fill(Color32::GRAY))
+                .show(ui, |ui| {
+                    ui.label("CentralPanel");
+                });
+        });
+    harness.run();
+    harness
+}
+
+fn hover_resize_handle(harness: &mut Harness<'_>) {
+    let outer = egui::PanelState::load(&harness.ctx, egui::Id::new(PANEL_ID))
+        .expect("PanelState should be persisted after the first frame")
+        .outer_rect;
+
+    // Hover just _inside_ the panel's inner edge, but still well within the resize grab radius:
+    // the `CentralPanel` and its label start exactly at that edge, and would otherwise take the
+    // hover from the resize handle.
+    harness.hover_at(Pos2::new(outer.center().x, outer.bottom() - 1.0));
+    harness.run();
+}
+
+#[test]
+fn separator_line_matrix() {
+    let mut results = SnapshotResults::new();
+
+    for show_separator_line in [false, true] {
+        let suffix = if show_separator_line { "on" } else { "off" };
+
+        // Not hovered: the line is dim (`show_separator_line`) or absent.
+        let mut harness = build_harness(show_separator_line);
+        results.add(harness.try_snapshot(format!("panel_separator_line/separator_{suffix}_idle")));
+
+        // Hovered: a `resizable` panel shows a bright line regardless of `show_separator_line`,
+        // and must not shift its contents to make room for it.
+        let mut harness = build_harness(show_separator_line);
+        hover_resize_handle(&mut harness);
+        results
+            .add(harness.try_snapshot(format!("panel_separator_line/separator_{suffix}_hovered")));
+    }
+}