docking: one drop preview painter for docking and panes

chrome::drop_preview draws the accent outline, filled or not, for every
docking destination and drag ghost and for the pane host's drop preview.

crates/ui/src/docking/chrome.rs

@@ -245,6 +245,13 @@ impl<'a> FloatingFrame<'a> {
     }
 }
 
+/// A drop preview: a 2 pt accent outline inside `rect`, filled with 15% accent when `filled`.
+/// Docking and the pane host draw every destination and drag ghost with it.
+pub fn drop_preview(painter: &egui::Painter, rect: Rect, accent: Color32, filled: bool) {
+    let fill = if filled { accent.gamma_multiply(0.15) } else { Color32::TRANSPARENT };
+    painter.rect(rect, 0.0, fill, Stroke::new(2.0, accent), StrokeKind::Inside);
+}
+
 /// Keep `rect` inside `bounds`, shrinking it when it is larger.
 pub fn clamp(rect: Rect, bounds: Rect) -> Rect {
     if !super::targets::usable(bounds) {

crates/ui/src/docking/render/preview.rs

@@ -1,8 +1,9 @@
 use std::{fmt::Debug, hash::Hash};
 
-use egui::{Color32, Context, Key, Pos2, Rect, Stroke, StrokeKind, Ui, Vec2, pos2, vec2};
+use egui::{Context, Key, Pos2, Rect, Ui, Vec2, pos2, vec2};
 
 use super::{Renderer, clamp_float, dock::group_action};
+use crate::docking::chrome::drop_preview;
 use crate::docking::{
     Action, Drag, DropProposal, DropRequest, GroupDrag, GroupGesture, Placement, Target, Zone, cancel_drag, dock_model::Place, resolve_drop,
     surface::TAB_PADDING,
@@ -137,7 +138,7 @@ impl<P: Clone + Eq + Hash + Debug + Send + Sync + 'static> Renderer<'_, P> {
         let painter = self.preview_painter(ctx);
         if drag.gesture != GroupGesture::Window {
             let ghost = self.group_ghost(ctx, drag, pointer);
-            painter.rect(ghost, 0.0, self.style.accent.gamma_multiply(0.15), Stroke::new(2.0, self.style.accent), StrokeKind::Inside);
+            drop_preview(&painter, ghost, self.style.accent, true);
             let strip = Rect::from_min_size(ghost.min, vec2(ghost.width(), self.style.tab_height.min(ghost.height())));
             let labels = painter.with_clip_rect(strip.shrink(2.0));
             let mut x = strip.left() + TAB_PADDING * 0.5;
@@ -164,9 +165,8 @@ impl<P: Clone + Eq + Hash + Debug + Send + Sync + 'static> Renderer<'_, P> {
         }
         if let Some(proposal) = self.group_proposal(drag, pointer).filter(|p| !p.noop) {
             let split = matches!(proposal.placement, Placement::Split(zone) | Placement::Edge(zone) if zone != Zone::Center);
-            let fill = if split { self.style.accent.gamma_multiply(0.15) } else { Color32::TRANSPARENT };
             if crate::docking::targets::usable(proposal.preview) {
-                painter.rect(proposal.preview.shrink(1.0), 0.0, fill, Stroke::new(2.0, self.style.accent), StrokeKind::Inside);
+                drop_preview(&painter, proposal.preview.shrink(1.0), self.style.accent, split);
             }
             if let Some(marker) = proposal.insertion {
                 painter.rect_filled(marker, 0.0, self.style.accent);
@@ -256,19 +256,13 @@ impl<P: Clone + Eq + Hash + Debug + Send + Sync + 'static> Renderer<'_, P> {
             let same_group = proposal.insertion.is_some()
                 && self.targets.iter().any(|target| target.rect == proposal.preview && target.tabs.iter().any(|(panel, _)| panel == &drag.panel));
             if !same_group && !proposal.noop && crate::docking::targets::usable(proposal.preview) {
-                painter.rect(
-                    proposal.preview.shrink(1.0),
-                    0.0,
-                    self.style.accent.gamma_multiply(0.15),
-                    Stroke::new(2.0, self.style.accent),
-                    StrokeKind::Inside,
-                );
+                drop_preview(&painter, proposal.preview.shrink(1.0), self.style.accent, true);
             }
             if let Some(marker) = proposal.insertion {
                 painter.rect_filled(marker, 0.0, self.style.accent);
             }
         } else if let Some(rect) = floating_rect {
-            painter.rect_stroke(rect, 0.0, Stroke::new(2.0, self.style.accent), StrokeKind::Inside);
+            drop_preview(&painter, rect, self.style.accent, false);
         }
     }
 
@@ -348,7 +342,7 @@ impl<P: Clone + Eq + Hash + Debug + Send + Sync + 'static> Renderer<'_, P> {
         if !released {
             let painter = self.preview_painter(ctx);
             if let Some(rect) = feedback.preview.filter(|rect| crate::docking::targets::usable(*rect)) {
-                painter.rect(rect, 0.0, self.style.accent.gamma_multiply(0.15), Stroke::new(2.0, self.style.accent), StrokeKind::Inside);
+                drop_preview(&painter, rect, self.style.accent, true);
             }
             if let Some(rect) = feedback.insertion.filter(|rect| crate::docking::targets::usable(*rect)) {
                 painter.rect_filled(rect, 0.0, self.style.accent);

crates/ui/src/panes/mod.rs

@@ -11,10 +11,7 @@ use std::fmt::Debug;
 use std::hash::Hash;
 use std::ops::RangeInclusive;
 
-use egui::{
-    Color32, Context, Frame, Id, Key, LayerId, Margin, Modifiers, Order, Pos2, Rect, Response, Sense, Stroke, StrokeKind, Ui, UiBuilder, Vec2, pos2,
-    vec2,
-};
+use egui::{Context, Frame, Id, Key, LayerId, Margin, Modifiers, Order, Pos2, Rect, Response, Sense, Ui, UiBuilder, Vec2, pos2, vec2};
 
 use crate::docking::PanelLimits;
 use crate::docking::chrome::{self, ChromeStyle, FloatingFrame, MoveStep, TitleBar};
@@ -599,7 +596,7 @@ impl PaneHost {
             Some(_) => Rect::from_x_y_ranges(bounds.right() - rect.width()..=bounds.right(), bounds.y_range()),
             None => float,
         };
-        paint_preview(preview, shape, style.accent);
+        chrome::drop_preview(preview, shape.shrink(1.0), style.accent, true);
     }
 
     #[allow(clippy::too_many_arguments)]
@@ -714,10 +711,6 @@ impl PaneHost {
     }
 }
 
-fn paint_preview(painter: &egui::Painter, rect: Rect, accent: Color32) {
-    painter.rect(rect.shrink(1.0), 0.0, accent.gamma_multiply(0.15), Stroke::new(2.0, accent), StrokeKind::Inside);
-}
-
 fn rect_array(rect: Rect) -> [f32; 4] {
     [rect.min.x, rect.min.y, rect.width(), rect.height()]
 }

docs/ui-docking.md

@@ -131,7 +131,8 @@ by the app's own Close items and commands.
 
 `DockArea::chrome_has_focus` tells whether keyboard focus is on docking chrome rather than on a
 panel body. `begin_drag` hands a drag that started in the app's own chrome to the area.
-`docking::chrome` has the floating frame and title bar for apps that draw their own windows.
+`docking::chrome` has the floating frame, the title bar and `drop_preview` (the drop preview
+docking and `craft_ui::panes` both draw) for apps that draw their own windows.
 
 ## Gestures