From 97213463878c84e23f6713892585398f3621eb4b Mon Sep 17 00:00:00 2001
From: Kovid Goyal <kovid@kovidgoyal.net>
Date: Sun, 29 Mar 2026 14:43:48 +0530
Subject: [PATCH] XWayland: Fix a regression where some wheel mice were not
 scrolling properly

We assume increment == 1 and delta * 120 == integer means we have
V120 events in units of 1/120.

Fixes #9770
---
 docs/changelog.rst  |  2 ++
 glfw/x11_platform.h |  1 +
 glfw/x11_window.c   | 16 ++++++++++++----
 3 files changed, 15 insertions(+), 4 deletions(-)

diff --git a/docs/changelog.rst b/docs/changelog.rst
index 0bde3d5ad..8e5128aa9 100644
--- a/docs/changelog.rst
+++ b/docs/changelog.rst
@@ -187,6 +187,8 @@ Detailed list of changes
 
 - X11: Fix a regression in the previous release that caused an occasional crash on input device removal (:iss:`9723`)
 
+- XWayland: Fix a regression where some wheel mice were not scrolling properly (:pull:`9770`)
+
 - Command palette: Improve searching to use word level matching (:pull:`9727`)
 
 - hints kitten: A new option to set the background color of matched text (:pull:`9745`)
diff --git a/glfw/x11_platform.h b/glfw/x11_platform.h
index fe78d17c8..d44a44f4e 100644
--- a/glfw/x11_platform.h
+++ b/glfw/x11_platform.h
@@ -247,6 +247,7 @@ typedef struct XIScrollDevice {
     char name[32];
     unsigned num_events;
     GLFWOffsetType offset_type;
+    bool v120_offset_needs_scaling;
 } XIScrollDevice;
 
 typedef struct XdndSelectionRequest {
diff --git a/glfw/x11_window.c b/glfw/x11_window.c
index 6be8e14ca..1924f8abf 100644
--- a/glfw/x11_window.c
+++ b/glfw/x11_window.c
@@ -1466,15 +1466,17 @@ handle_xi_motion_event(_GLFWwindow *window, XIDeviceEvent *de) {
             *off = delta;
             d->num_events++;
             if (!d->type_detected) {
+                debug_input("Detecting scroll device type: delta: %.3f delta*120: %.3f v->increment: %.3f\n", delta, delta * 120, v->increment);
                 if (v->increment == 120.) {
                     d->type_detected = true;
                     d->offset_type = GLFW_SCROLL_OFFEST_V120;
+                    d->v120_offset_needs_scaling = false;
                 } else {
-                    bool delta_is_fractional = number_has_fractional_part(delta);
-                    if (delta_is_fractional) {
+                    if (number_has_fractional_part(delta)) {
                         if (fabs(delta * 120 - round(delta * 120)) < 0.01) {
                             d->type_detected = d->num_events > 2;
                             d->offset_type = GLFW_SCROLL_OFFEST_V120;
+                            d->v120_offset_needs_scaling = v->increment == 1.;
                         } else {
                             d->type_detected = true;
                             d->offset_type = GLFW_SCROLL_OFFEST_HIGHRES;
@@ -1485,8 +1487,14 @@ handle_xi_motion_event(_GLFWwindow *window, XIDeviceEvent *de) {
                     }
                 }
             }
-            if (d->offset_type == GLFW_SCROLL_OFFSET_LINES) {
-                if (v->increment != 0) *off /= v->increment;
+            if (v->increment != 0) {
+                if (d->offset_type == GLFW_SCROLL_OFFSET_LINES) {
+                    *off /= v->increment;
+                } else if (d->offset_type == GLFW_SCROLL_OFFEST_V120 && d->v120_offset_needs_scaling) {
+                    // On XWayland, scroll deltas are in scroll-increment units (typically
+                    // where increment=1.0 means one line or one v120).
+                    *off *= 120.;
+                }
             }
         }
         type = d->offset_type;