From 84f5d9c39320e3676c102dd49df93c76bb826c35 Mon Sep 17 00:00:00 2001
From: Jameson Ernst <jameson@jpernst.com>
Date: Wed, 14 Sep 2022 22:11:32 -0700
Subject: [PATCH] Improve handling of High-DPI mice  - Reduce lower bound of
 mouse sensitivity  - Add error accumulators to prevent small motions from
 being rounded off

---
 src/config.c                |  4 ++--
 src/include/86box/86box.h   |  1 +
 src/qt/qt_progsettings.ui   |  2 +-
 src/qt/qt_rendererstack.cpp | 11 +++++++++--
 src/unix/unix_sdl.c         |  1 +
 src/win/win_mouse.c         |  1 +
 6 files changed, 15 insertions(+), 5 deletions(-)

diff --git a/src/config.c b/src/config.c
index 01710ebc4..69c4e7c6b 100644
--- a/src/config.c
+++ b/src/config.c
@@ -180,8 +180,8 @@ load_general(void)
         lang_id = plat_language_code(p);
 
     mouse_sensitivity = ini_section_get_double(cat, "mouse_sensitivity", 1.0);
-    if (mouse_sensitivity < 0.5)
-        mouse_sensitivity = 0.5;
+    if (mouse_sensitivity < 0.1)
+        mouse_sensitivity = 0.1;
     else if (mouse_sensitivity > 2.0)
         mouse_sensitivity = 2.0;
 
diff --git a/src/include/86box/86box.h b/src/include/86box/86box.h
index fce923e4f..4f9ccabed 100644
--- a/src/include/86box/86box.h
+++ b/src/include/86box/86box.h
@@ -136,6 +136,7 @@ extern int is_pentium; /* TODO: Move back to cpu/cpu.h when it's figured out,
                                 how to remove that hack from the ET4000/W32p. */
 extern int    fixed_size_x, fixed_size_y;
 extern double mouse_sensitivity; /* (C) Mouse sensitivity scale */
+extern double mouse_x_error, mouse_y_error;    /* Mouse error accumulators */
 extern int    pit_mode;          /* (C) force setting PIT mode */
 extern int    fm_driver;         /* (C) select FM sound driver */
 
diff --git a/src/qt/qt_progsettings.ui b/src/qt/qt_progsettings.ui
index 05775489f..fa0818652 100644
--- a/src/qt/qt_progsettings.ui
+++ b/src/qt/qt_progsettings.ui
@@ -88,7 +88,7 @@
    <item row="7" column="0" colspan="2">
     <widget class="QSlider" name="horizontalSlider">
      <property name="minimum">
-      <number>50</number>
+      <number>10</number>
      </property>
      <property name="maximum">
       <number>200</number>
diff --git a/src/qt/qt_rendererstack.cpp b/src/qt/qt_rendererstack.cpp
index 6beb37516..f0e5b12e0 100644
--- a/src/qt/qt_rendererstack.cpp
+++ b/src/qt/qt_rendererstack.cpp
@@ -54,6 +54,7 @@ extern "C" {
 #include <86box/video.h>
 
 double mouse_sensitivity = 1.0;
+double mouse_x_error = 0.0, mouse_y_error = 0.0;
 }
 
 struct mouseinputdata {
@@ -151,8 +152,14 @@ RendererStack::mousePoll()
 #endif
         this->mouse_poll_func();
 
-    mouse_x *= mouse_sensitivity;
-    mouse_y *= mouse_sensitivity;
+    double scaled_x = mouse_x * mouse_sensitivity + mouse_x_error;
+    double scaled_y = mouse_y * mouse_sensitivity + mouse_y_error;
+
+    mouse_x = static_cast<int>(scaled_x);
+    mouse_y = static_cast<int>(scaled_y);
+
+    mouse_x_error = scaled_x - mouse_x;
+    mouse_y_error = scaled_y - mouse_y;
 }
 
 int ignoreNextMouseEvent = 1;
diff --git a/src/unix/unix_sdl.c b/src/unix/unix_sdl.c
index 3d989bf53..b023299d2 100644
--- a/src/unix/unix_sdl.c
+++ b/src/unix/unix_sdl.c
@@ -44,6 +44,7 @@ int resize_pending = 0;
 int resize_w = 0;
 int resize_h = 0;
 double mouse_sensitivity = 1.0; /* Unused. */
+double mouse_x_error = 0.0, mouse_y_error = 0.0; /* Unused. */
 static uint8_t interpixels[17842176];
 
 extern void RenderImGui();
diff --git a/src/win/win_mouse.c b/src/win/win_mouse.c
index bb592f419..dfc0ac691 100644
--- a/src/win/win_mouse.c
+++ b/src/win/win_mouse.c
@@ -29,6 +29,7 @@
 
 int    mouse_capture;
 double mouse_sensitivity = 1.0; /* Unused. */
+double mouse_x_error = 0.0, mouse_y_error = 0.0; /* Unused. */
 
 typedef struct {
     int buttons;