From 4daff7300f502e4ee56a847608959e52bb8a49bd Mon Sep 17 00:00:00 2001
From: galister <22305755+galister@users.noreply.github.com>
Date: Fri, 3 Jul 2026 00:32:56 +0900
Subject: [PATCH] fix scale drift when using align_to_hmd

---
 wayvr/src/backend/input.rs    |  8 +++++---
 wayvr/src/windowing/mod.rs    |  4 +++-
 wayvr/src/windowing/window.rs | 28 +++++++++++++++++++++-------
 3 files changed, 29 insertions(+), 11 deletions(-)

diff --git a/wayvr/src/backend/input.rs b/wayvr/src/backend/input.rs
index 88e8738b..6fb658f8 100644
--- a/wayvr/src/backend/input.rs
+++ b/wayvr/src/backend/input.rs
@@ -20,7 +20,7 @@ use crate::subsystem::hid::WheelDelta;
 use crate::subsystem::input::KeyboardFocus;
 use crate::windowing::backend::OverlayEventData;
 use crate::windowing::manager::OverlayWindowManager;
-use crate::windowing::window::{self, OverlayWindowData, realign};
+use crate::windowing::window::{self, OverlayWindowData, realign, scalar_scale, window_scale};
 use crate::windowing::{OverlayID, OverlaySelector};
 
 use super::task::TaskType;
@@ -825,7 +825,8 @@ where
             } else {
                 app.anchor.translation =
                     pointer.pose.transform_point3a(grab_data.offset.translation);
-                realign(&mut app.anchor, &app.input_state.hmd);
+                let scale = scalar_scale(&app.anchor);
+                realign(&mut app.anchor, &app.input_state.hmd, scale);
             }
         } else {
             // single grab resize
@@ -846,9 +847,10 @@ where
             if pointer.now.click_modifier_right {
                 overlay_state.transform = pointer.pose * grab_data.offset;
             } else {
+                let scale = window_scale(overlay_state);
                 overlay_state.transform.translation =
                     pointer.pose.transform_point3a(grab_data.offset.translation);
-                realign(&mut overlay_state.transform, &app.input_state.hmd);
+                realign(&mut overlay_state.transform, &app.input_state.hmd, scale);
             }
             overlay.config.pause_movement = true;
             overlay.config.dirty = true;
diff --git a/wayvr/src/windowing/mod.rs b/wayvr/src/windowing/mod.rs
index 2e820cfd..5c11272e 100644
--- a/wayvr/src/windowing/mod.rs
+++ b/wayvr/src/windowing/mod.rs
@@ -2,6 +2,8 @@ use glam::{Affine3A, Vec3A};
 use slotmap::new_key_type;
 use std::sync::Arc;
 
+use crate::windowing::window::scalar_scale;
+
 pub mod backend;
 pub mod manager;
 pub mod set;
@@ -28,7 +30,7 @@ pub const Z_ORDER_DASHBOARD: u32 = Z_ORDER_DEFAULT;
 
 pub fn snap_upright(transform: Affine3A, up_dir: Vec3A) -> Affine3A {
     if transform.x_axis.dot(up_dir).abs() < 0.2 {
-        let scale = transform.x_axis.length();
+        let scale = scalar_scale(&transform);
         let col_z = transform.z_axis.normalize();
         let col_y = up_dir;
         let col_x = col_y.cross(col_z);
diff --git a/wayvr/src/windowing/window.rs b/wayvr/src/windowing/window.rs
index ebd84fae..6d025821 100644
--- a/wayvr/src/windowing/window.rs
+++ b/wayvr/src/windowing/window.rs
@@ -146,6 +146,7 @@ impl OverlayWindowConfig {
         let cur_transform = state
             .saved_transform
             .unwrap_or(self.default_state.transform);
+        let scale = scalar_scale(&cur_transform);
 
         let (parent_transform, lerp, align_to_hmd) = match state.positioning {
             Positioning::FollowHead { lerp } => (app.input_state.hmd, lerp, false),
@@ -167,8 +168,6 @@ impl OverlayWindowConfig {
         state.transform = match lerp {
             1.0 => target_transform,
             lerp => {
-                let scale = target_transform.matrix3.x_axis.length();
-
                 let rot_from = Quat::from_mat3a(&state.transform.matrix3.div_scalar(scale));
                 let rot_to = Quat::from_mat3a(&target_transform.matrix3.div_scalar(scale));
 
@@ -187,7 +186,7 @@ impl OverlayWindowConfig {
         };
 
         if align_to_hmd {
-            realign(&mut state.transform, &app.input_state.hmd);
+            realign(&mut state.transform, &app.input_state.hmd, scale);
         }
 
         self.dirty = true;
@@ -244,13 +243,14 @@ impl OverlayWindowConfig {
         state.transform = parent_transform * cur_transform;
 
         if align_to_hmd || (state.grabbable && hard_reset) {
-            realign(&mut state.transform, &app.input_state.hmd);
+            let scale = scalar_scale(&cur_transform);
+            realign(&mut state.transform, &app.input_state.hmd, scale);
         }
         self.dirty = true;
     }
 }
 
-pub fn realign(transform: &mut Affine3A, hmd: &Affine3A) {
+pub fn realign(transform: &mut Affine3A, hmd: &Affine3A, scale: f32) {
     let to_hmd = hmd.translation - transform.translation;
     let up_dir: Vec3A;
 
@@ -277,8 +277,6 @@ pub fn realign(transform: &mut Affine3A, hmd: &Affine3A) {
         }
     }
 
-    let scale = transform.x_axis.length();
-
     let col_z = (transform.translation - hmd.translation).normalize();
     let col_y = up_dir;
     let col_x = col_y.cross(col_z);
@@ -289,6 +287,22 @@ pub fn realign(transform: &mut Affine3A, hmd: &Affine3A) {
     transform.matrix3 = Mat3A::from_cols(col_x, col_y, col_z).mul_scalar(scale) * rot;
 }
 
+pub fn window_scale(state: &OverlayWindowState) -> f32 {
+    state
+        .saved_transform
+        .as_ref()
+        .map(scalar_scale)
+        .unwrap_or_else(|| scalar_scale(&state.transform))
+}
+
+pub fn scalar_scale(a: &Affine3A) -> f32 {
+    let det = a.matrix3.determinant();
+    (a.matrix3.x_axis.length() * det.signum()
+        + a.matrix3.y_axis.length()
+        + a.matrix3.z_axis.length())
+        / 3.0
+}
+
 pub fn save_transform(state: &mut OverlayWindowState, app: &mut AppState) {
     let parent_transform = match state.positioning {
         Positioning::Floating => snap_upright(app.input_state.hmd, Vec3A::Y),