feat(ui): anchor every ribbon dropdown and status-bar popup to its widget, flipping at window edges

All ribbon dropdowns (tool dropdowns, undo/redo history, and the layer /
colour / linetype / lineweight / style combos) and all status-bar popups
(scale, units, OSNAP, isolate, filter, customization menu, layout list)
now open directly at their widget instead of at a layout-estimated
position, and flip to the other side when the panel would cross a window
edge.

Each dropdown/pill widget is wrapped in a transparent PosReport that
records its screen bounds into a thread-local map on draw. The overlays
read those bounds:
- Ribbon: Ribbon::dd_anchor(id, panel_w, win_w) returns
  (align_right, h_pad, top); position_ribbon_dropdown places the panel
  below the widget, flipping right-aligned near the right edge.
- Status bar: popup builders take (pill_bounds, win_size) and call
  popup::position_statusbar_popup / popup_anchor, opening just above the
  pill and flipping left/right at the window edge.

The window size is now tracked (win_size, set on WindowResized). The old
layout estimators compute_dropdown_left / compute_layer_combo_left /
compute_prop_combo_left / compute_history_dropdown_left and
osnap_popup_right_offset are deleted.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/ui/popup/mod.rs

@@ -4,3 +4,76 @@ pub mod scale_popup;
 pub mod selection_filter_popup;
 pub mod snap_popup;
 pub mod units_popup;
+
+use iced::widget::container;
+use iced::{Element, Fill, Padding, Rectangle};
+
+use crate::app::Message;
+
+/// Decide a status-bar popup's horizontal anchor and paddings from its pill's
+/// screen bounds. Returns `(align_right, horizontal_pad, bottom_pad)`.
+///
+/// The popup opens just above the pill. Horizontally it prefers `prefer_right`
+/// (right edge aligned to the pill's right, growing left) but flips to the other
+/// side when that direction would cross the window edge.
+pub fn popup_anchor(
+    pill: Option<Rectangle>,
+    win: (f32, f32),
+    popup_w: f32,
+    prefer_right: bool,
+) -> (bool, f32, f32) {
+    let Some(b) = pill else {
+        // No recorded position yet — fall back to the bottom-right corner.
+        return (prefer_right, 4.0, 27.0);
+    };
+    let bottom = (win.1 - b.y).max(0.0);
+    let right_pad = (win.0 - (b.x + b.width)).max(4.0);
+    let left_pad = b.x.max(4.0);
+
+    if prefer_right {
+        // Right-aligned popup grows left; flip left-aligned if it would pass the
+        // left window edge.
+        if (b.x + b.width) - popup_w < 2.0 {
+            (false, left_pad, bottom)
+        } else {
+            (true, right_pad, bottom)
+        }
+    } else {
+        // Left-aligned popup grows right; flip right-aligned if it would pass the
+        // right window edge.
+        if b.x + popup_w > win.0 - 2.0 {
+            (true, right_pad, bottom)
+        } else {
+            (false, left_pad, bottom)
+        }
+    }
+}
+
+/// Wrap a status-bar popup `panel` in a full-window container positioned just
+/// above its pill, flipping horizontally to stay on screen.
+pub fn position_statusbar_popup<'a>(
+    panel: Element<'a, Message>,
+    pill: Option<Rectangle>,
+    win: (f32, f32),
+    popup_w: f32,
+    prefer_right: bool,
+) -> Element<'a, Message> {
+    let (align_right, h_pad, bottom) = popup_anchor(pill, win, popup_w, prefer_right);
+    let pad = Padding {
+        top: 0.0,
+        bottom,
+        left: if align_right { 0.0 } else { h_pad },
+        right: if align_right { h_pad } else { 0.0 },
+    };
+    let c = container(panel)
+        .align_bottom(Fill)
+        .width(Fill)
+        .height(Fill)
+        .padding(pad);
+    let c = if align_right {
+        c.align_right(Fill)
+    } else {
+        c.align_left(Fill)
+    };
+    c.into()
+}