fix(clipboard): paste anchor at bounding-box lower-left, not center (#285)

COPYCLIP/CUTCLIP used the mean of each entity's bounding-box centre as the
paste anchor. Standard clipboard behaviour puts the lower-left corner of the
box enclosing all copied items under the cursor. Union each entity's bbox min
corner instead, and rename clipboard_centroid -> clipboard_base (COPYBASE also
stores a picked point there, so "centroid" was inaccurate).

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/app/command_driver.rs

@@ -790,7 +790,7 @@ impl OpenCADStudio {
                 if self.clipboard.is_empty() {
                     self.command_line.push_error("Clipboard is empty.");
                 } else {
-                    let delta = base_pt - self.clipboard_centroid;
+                    let delta = base_pt - self.clipboard_base;
                     let translate = crate::command::EntityTransform::Translate(delta);
                     self.push_undo_snapshot(i, "PASTECLIP");
                     let count = self.clipboard.len();

src/app/commands/blocks.rs

@@ -70,7 +70,7 @@ impl OpenCADStudio {
                         .iter()
                         .filter_map(|&h| self.tabs[i].scene.document.get_entity(h).cloned())
                         .collect();
-                    self.clipboard_centroid = super::super::helpers::entities_centroid_by_bbox(
+                    self.clipboard_base = super::super::helpers::entities_lower_left_by_bbox(
                         &self.tabs[i].scene.document,
                         &handles,
                     );
@@ -87,7 +87,7 @@ impl OpenCADStudio {
             }
 
             // COPYBASE — copy the selection to the clipboard with a picked base
-            // point (vs COPYCLIP, which uses the selection centroid).
+            // point (vs COPYCLIP, which uses the selection's lower-left corner).
             "COPYBASE" => {
                 let handles: Vec<_> = self.tabs[i]
                     .scene
@@ -127,7 +127,7 @@ impl OpenCADStudio {
                         .iter()
                         .filter_map(|&h| self.tabs[i].scene.document.get_entity(h).cloned())
                         .collect();
-                    self.clipboard_centroid = base;
+                    self.clipboard_base = base;
                     self.clipboard = entities;
                     self.clipboard_deps = super::super::ClipboardDeps::capture(
                         &self.tabs[i].scene.document,
@@ -159,7 +159,7 @@ impl OpenCADStudio {
                         .iter()
                         .filter_map(|&h| self.tabs[i].scene.document.get_entity(h).cloned())
                         .collect();
-                    self.clipboard_centroid = super::super::helpers::entities_centroid_by_bbox(
+                    self.clipboard_base = super::super::helpers::entities_lower_left_by_bbox(
                         &self.tabs[i].scene.document,
                         &handles,
                     );
@@ -184,11 +184,11 @@ impl OpenCADStudio {
                     self.command_line.push_error("Clipboard is empty.");
                 } else {
                     let wires = self.tabs[i].scene.wires_for_entities(&self.clipboard);
-                    let centroid = self.clipboard_centroid;
+                    let base = self.clipboard_base;
                     use crate::modules::draw::clipboard::paste::PasteCommand;
                     // The ghost anchor is a display-only offset; the precise
                     // paste delta is computed in f64 at commit time.
-                    let cmd = PasteCommand::new(wires, centroid.as_vec3());
+                    let cmd = PasteCommand::new(wires, base.as_vec3());
                     self.command_line.push_info(&cmd.prompt());
                     self.tabs[i].active_cmd = Some(Box::new(cmd));
                 }
@@ -232,7 +232,7 @@ impl OpenCADStudio {
                     // block's nested insert keeps its clip. (#xclip-paste)
                     let ext_roots = self.recreate_clipboard_ext_roots(i);
                     let name = self.unique_block_name("Block");
-                    let base = self.clipboard_centroid;
+                    let base = self.clipboard_base;
                     let mut entities = self.clipboard.clone();
                     for (idx, root) in ext_roots {
                         if let Some(e) = entities.get_mut(idx) {
@@ -246,7 +246,7 @@ impl OpenCADStudio {
                         Ok(()) => {
                             // Block defined; now place it interactively so the
                             // user picks the drop point (insertion uses the
-                            // clipboard centroid as the block's base). The
+                            // clipboard lower-left corner as the block's base). The
                             // clipboard wires rubber-band under the cursor.
                             self.tabs[i].scene.populate_meshes_from_document();
                             self.tabs[i].dirty = true;

src/app/helpers.rs

@@ -252,32 +252,31 @@ pub(super) fn polar_constrain_near(
 
 // ── Clipboard / selection helpers ──────────────────────────────────────────
 
-/// Cheap copy/paste anchor: the mean of each entity's bounding-box centre.
-/// Replaces averaging every tessellated wire vertex, which cost O(total
-/// geometry) and stalled a whole-drawing copy. The anchor only sets which point
-/// of the selection sits under the cursor, so a per-entity bbox centre is an
-/// equally valid, far cheaper origin.
-pub(super) fn entities_centroid_by_bbox(
+/// Copy/paste anchor: the lower-left corner of the bounding box that encloses
+/// every copied entity. Standard clipboard behaviour puts this corner under the
+/// cursor at paste time, so the whole selection drops down-and-right of the pick.
+///
+/// Unions each entity's own bounding box (O(entities)) rather than averaging
+/// every tessellated wire vertex, which cost O(total geometry) and stalled a
+/// whole-drawing copy. The per-entity `min` corners give the exact enclosing
+/// box's lower-left.
+pub(super) fn entities_lower_left_by_bbox(
     doc: &acadrust::CadDocument,
     handles: &[acadrust::Handle],
 ) -> glam::DVec3 {
-    let mut sum = glam::DVec3::ZERO;
-    let mut count = 0usize;
+    let mut min = glam::DVec3::splat(f64::INFINITY);
+    let mut any = false;
     for &h in handles {
         let Some(e) = doc.get_entity(h) else { continue };
         let bb = e.as_entity().bounding_box();
-        let c = glam::DVec3::new(
-            (bb.min.x + bb.max.x) * 0.5,
-            (bb.min.y + bb.max.y) * 0.5,
-            (bb.min.z + bb.max.z) * 0.5,
-        );
-        if c.x.is_finite() && c.y.is_finite() && c.z.is_finite() {
-            sum += c;
-            count += 1;
+        let lo = glam::DVec3::new(bb.min.x, bb.min.y, bb.min.z);
+        if lo.x.is_finite() && lo.y.is_finite() && lo.z.is_finite() {
+            min = min.min(lo);
+            any = true;
         }
     }
-    if count > 0 {
-        sum / count as f64
+    if any {
+        min
     } else {
         glam::DVec3::ZERO
     }

src/app/mod.rs

@@ -500,8 +500,10 @@ pub(super) struct OpenCADStudio {
     clipboard: Vec<acadrust::EntityType>,
     /// Entities removed by the most recent ERASE, kept so OOPS can restore them.
     oops_cache: Vec<acadrust::EntityType>,
-    /// Centroid of the clipboard entities (world XY plane).
-    clipboard_centroid: glam::DVec3,
+    /// Paste anchor: lower-left corner of the clipboard entities' bounding box
+    /// (or the point picked by COPYBASE). This point lands under the cursor at
+    /// paste time.
+    clipboard_base: glam::DVec3,
     /// Table records (layer / linetype / text + dim style) the clipboard
     /// entities reference, captured from the source drawing at copy time so a
     /// paste into a *different* drawing can recreate any that are missing —
@@ -2152,7 +2154,7 @@ impl OpenCADStudio {
             update_notice_body: None,
             clipboard: Vec::new(),
             oops_cache: Vec::new(),
-            clipboard_centroid: glam::DVec3::ZERO,
+            clipboard_base: glam::DVec3::ZERO,
             clipboard_deps: ClipboardDeps::default(),
             shift_down: false,
             ctrl_down: false,

src/modules/draw/clipboard/copy_base.rs

@@ -1,6 +1,6 @@
 // COPYBASE — copy the current selection to the clipboard using a base point the
-// user picks (rather than the selection's centroid, as COPYCLIP does). The
-// command only collects the base point; the host performs the copy when it
+// user picks (rather than the selection's lower-left corner, as COPYCLIP does).
+// The command only collects the base point; the host performs the copy when it
 // receives the dispatched `COPYBASE_AT <x> <y> <z>` token.
 
 use glam::DVec3;

src/modules/draw/clipboard/paste.rs

@@ -32,8 +32,9 @@ pub struct PasteCommand {
     /// just this 5-point rectangle instead — O(1) per frame. `None` = ghost the
     /// full wires.
     bbox_wire: Option<WireModel>,
-    /// Centroid of the clipboard entities (offset origin for translation).
-    centroid: Vec3,
+    /// Paste anchor — lower-left corner of the clipboard entities (offset origin
+    /// for translation; the point that tracks the cursor).
+    base: Vec3,
 }
 
 impl PasteCommand {
@@ -44,17 +45,17 @@ impl PasteCommand {
     const MAX_PREVIEW_WIRES: usize = 20_000;
     const MAX_PREVIEW_POINTS: usize = 300_000;
 
-    pub fn new(wires: Vec<WireModel>, centroid: Vec3) -> Self {
+    pub fn new(wires: Vec<WireModel>, base: Vec3) -> Self {
         let total_points: usize = wires.iter().map(|w| w.points.len()).sum();
         let too_heavy =
             wires.len() > Self::MAX_PREVIEW_WIRES || total_points > Self::MAX_PREVIEW_POINTS;
         if too_heavy {
             if let Some(bbox_wire) = Self::bbox_outline(&wires) {
                 // Drop the full wires — the box is all the ghost needs now.
-                return Self { wires: Vec::new(), bbox_wire: Some(bbox_wire), centroid };
+                return Self { wires: Vec::new(), bbox_wire: Some(bbox_wire), base };
             }
         }
-        Self { wires, bbox_wire: None, centroid }
+        Self { wires, bbox_wire: None, base }
     }
 
     /// Build a closed rectangle outline around the XY extent of `wires`, in the
@@ -119,7 +120,7 @@ impl CadCommand for PasteCommand {
 
     fn on_preview_wires(&mut self, pt: DVec3) -> Vec<WireModel> {
         let pt = pt.as_vec3();
-        let delta = pt - self.centroid;
+        let delta = pt - self.base;
         // Large clipboard: ghost just the bounding box — O(1) per frame.
         if let Some(bbox) = &self.bbox_wire {
             return vec![bbox.translated(delta)];