Implement high precision scrolling with the trackpad on platforms such as macOS and Wayland that implement it.

Fixes #819. Note that I have no idea whether the code works well on
retina screens, might have to multiple the pixel count by the scale
factor on those screens?

docs/changelog.rst

@@ -20,6 +20,9 @@ Changelog
 - Add an option :opt:`env` to set environment variables in child processes
   from kitty.conf
 
+- Implement high precision scrolling with the trackpad on platforms such as
+  macOS and Wayland that implement it. (:pull:`819`)
+
 - macOS: Allow scrolling window contents using mouse wheel/trackpad even when the
   window is not the active window (:iss:`729`)
 

glfw/cocoa_window.m

@@ -884,15 +884,10 @@ is_ascii_control_char(char x) {
 
     deltaX = [event scrollingDeltaX];
     deltaY = [event scrollingDeltaY];
-
-    if ([event hasPreciseScrollingDeltas])
-    {
-        deltaX *= 0.1;
-        deltaY *= 0.1;
-    }
+    int flags = [event hasPreciseScrollingDeltas] ? 1 : 0;
 
     if (fabs(deltaX) > 0.0 || fabs(deltaY) > 0.0)
-        _glfwInputScroll(window, deltaX, deltaY);
+        _glfwInputScroll(window, deltaX, deltaY, flags);
 }
 
 - (NSDragOperation)draggingEntered:(id <NSDraggingInfo>)sender

glfw/glfw3.h

@@ -1372,15 +1372,17 @@ typedef void (* GLFWcursorenterfun)(GLFWwindow*,int);
  *  @param[in] window The window that received the event.
  *  @param[in] xoffset The scroll offset along the x-axis.
  *  @param[in] yoffset The scroll offset along the y-axis.
+ *  @param[in] flags A bit-mask providing extra data about the event. flags & 1 will be true if and only if the offset values are "high-precision". Typically pixel values. Otherwise the offset values are number of lines.
  *
  *  @sa @ref scrolling
  *  @sa @ref glfwSetScrollCallback
  *
  *  @since Added in version 3.0.  Replaces `GLFWmousewheelfun`.
+ *  @since Changed in version 4.0.  Added `flags` parameter.
  *
  *  @ingroup input
  */
-typedef void (* GLFWscrollfun)(GLFWwindow*,double,double);
+typedef void (* GLFWscrollfun)(GLFWwindow*,double,double,int);
 
 /*! @brief The function signature for key callbacks.
  *

glfw/input.c

@@ -286,10 +286,10 @@ void _glfwInputKeyboard(_GLFWwindow* window, int key, int scancode, int action,
 
 // Notifies shared code of a scroll event
 //
-void _glfwInputScroll(_GLFWwindow* window, double xoffset, double yoffset)
+void _glfwInputScroll(_GLFWwindow* window, double xoffset, double yoffset, int flags)
 {
     if (window->callbacks.scroll)
-        window->callbacks.scroll((GLFWwindow*) window, xoffset, yoffset);
+        window->callbacks.scroll((GLFWwindow*) window, xoffset, yoffset, flags);
 }
 
 // Notifies shared code of a mouse button click event

glfw/internal.h

@@ -717,7 +717,7 @@ void _glfwInputWindowCloseRequest(_GLFWwindow* window);
 void _glfwInputWindowMonitor(_GLFWwindow* window, _GLFWmonitor* monitor);
 
 void _glfwInputKeyboard(_GLFWwindow* window, int key, int scancode, int action, int mods, const char* text, int state);
-void _glfwInputScroll(_GLFWwindow* window, double xoffset, double yoffset);
+void _glfwInputScroll(_GLFWwindow* window, double xoffset, double yoffset, int flags);
 void _glfwInputMouseClick(_GLFWwindow* window, int button, int action, int mods);
 void _glfwInputCursorPos(_GLFWwindow* window, double xpos, double ypos);
 void _glfwInputCursorEnter(_GLFWwindow* window, GLFWbool entered);

glfw/wl_init.c

@@ -319,11 +319,6 @@ static void pointerHandleAxis(void* data,
 {
     _GLFWwindow* window = _glfw.wl.pointerFocus;
     double x = 0.0, y = 0.0;
-    // Wayland scroll events are in pointer motion coordinate space (think two
-    // finger scroll).  The factor 10 is commonly used to convert to "scroll
-    // step means 1.0.
-    const double scrollFactor = 1.0 / 10.0;
-
     if (!window)
         return;
 
@@ -331,11 +326,11 @@ static void pointerHandleAxis(void* data,
            axis == WL_POINTER_AXIS_VERTICAL_SCROLL);
 
     if (axis == WL_POINTER_AXIS_HORIZONTAL_SCROLL)
-        x = wl_fixed_to_double(value) * scrollFactor;
+        x = wl_fixed_to_double(value) * -1;
     else if (axis == WL_POINTER_AXIS_VERTICAL_SCROLL)
-        y = wl_fixed_to_double(value) * scrollFactor;
+        y = wl_fixed_to_double(value) * -1;
 
-    _glfwInputScroll(window, x, y);
+    _glfwInputScroll(window, x, y, 1);
 }
 
 static const struct wl_pointer_listener pointerListener = {

glfw/x11_window.c

@@ -1309,13 +1309,13 @@ static void processEvent(XEvent *event)
 
             // Modern X provides scroll events as mouse button presses
             else if (event->xbutton.button == Button4)
-                _glfwInputScroll(window, 0.0, 1.0);
+                _glfwInputScroll(window, 0.0, 1.0, 0);
             else if (event->xbutton.button == Button5)
-                _glfwInputScroll(window, 0.0, -1.0);
+                _glfwInputScroll(window, 0.0, -1.0, 0);
             else if (event->xbutton.button == Button6)
-                _glfwInputScroll(window, 1.0, 0.0);
+                _glfwInputScroll(window, 1.0, 0.0, 0);
             else if (event->xbutton.button == Button7)
-                _glfwInputScroll(window, -1.0, 0.0);
+                _glfwInputScroll(window, -1.0, 0.0, 0);
 
             else
             {

kitty/config_data.py

@@ -302,8 +302,9 @@ INPUT_LINE_NUMBER in the command line above will be replaced by an integer
 representing which line should be at the top of the screen.'''))
 
 o('wheel_scroll_multiplier', 5.0, long_text=_('''
-Modify the amount scrolled by the mouse wheel or touchpad. Use
-negative numbers to change scroll direction.'''))
+Modify the amount scrolled by the mouse wheel. Note this is only used for low
+precision scrolling devices, not for high precision scrolling on platforms such
+as macOS and Wayland. Use negative numbers to change scroll direction.'''))
 # }}}
 
 g('mouse')  # {{{

kitty/data-types.h

@@ -284,7 +284,7 @@ void enter_event();
 void mouse_event(int, int);
 void focus_in_event();
 void wakeup_io_loop(bool);
-void scroll_event(double, double);
+void scroll_event(double, double, int);
 void fake_scroll(int, bool);
 void set_special_key_combo(int glfw_key, int mods, bool is_native);
 void on_key_input(int key, int scancode, int action, int mods, const char*, int);

kitty/glfw-wrapper.h

@@ -1141,7 +1141,7 @@ typedef void (* GLFWcursorenterfun)(GLFWwindow*,int);
  *
  *  @ingroup input
  */
-typedef void (* GLFWscrollfun)(GLFWwindow*,double,double);
+typedef void (* GLFWscrollfun)(GLFWwindow*,double,double,int);
 
 /*! @brief The function signature for key callbacks.
  *

kitty/glfw.c

@@ -178,12 +178,12 @@ cursor_pos_callback(GLFWwindow *w, double x, double y) {
 }
 
 static void
-scroll_callback(GLFWwindow *w, double xoffset, double yoffset) {
+scroll_callback(GLFWwindow *w, double xoffset, double yoffset, int flags) {
     if (!set_callback_window(w)) return;
     show_mouse_cursor(w);
     double now = monotonic();
     global_state.callback_os_window->last_mouse_activity_at = now;
-    if (is_window_ready_for_callbacks()) scroll_event(xoffset, yoffset);
+    if (is_window_ready_for_callbacks()) scroll_event(xoffset, yoffset, flags);
     global_state.callback_os_window = NULL;
 }
 

kitty/mouse.c

@@ -527,13 +527,7 @@ mouse_event(int button, int modifiers) {
 }
 
 void
-scroll_event(double UNUSED xoffset, double yoffset) {
-    // glfw inverts the y-axis when reporting scroll events under wayland
-    // Until this is fixed in upstream, invert y ourselves.
-    if (global_state.is_wayland) yoffset *= -1;
-    int s = (int) round(yoffset * OPT(wheel_scroll_multiplier));
-    if (s == 0) return;
-    bool upwards = s > 0;
+scroll_event(double UNUSED xoffset, double yoffset, int flags) {
     bool in_tab_bar;
     unsigned int window_idx = 0;
     Window *w = window_for_event(&window_idx, &in_tab_bar);
@@ -544,17 +538,36 @@ scroll_event(double UNUSED xoffset, double yoffset) {
         Tab *t = global_state.callback_os_window->tabs + global_state.callback_os_window->active_tab;
         if (t) w = t->windows + t->active_window;
     }
-    if (w) {
-        Screen *screen = w->render_data.screen;
-        if (screen->linebuf == screen->main_linebuf) {
-            screen_history_scroll(screen, abs(s), upwards);
+    if (!w) return;
+
+    int s;
+    if (flags & 1) {
+        if (yoffset * global_state.callback_os_window->pending_scroll_pixels < 0) {
+            global_state.callback_os_window->pending_scroll_pixels = 0;  // change of direction
+        }
+        double pixels = global_state.callback_os_window->pending_scroll_pixels + yoffset;
+        if (fabs(pixels) < global_state.callback_os_window->fonts_data->cell_height) {
+            global_state.callback_os_window->pending_scroll_pixels = pixels;
+            return;
+        }
+        s = abs(((int)round(pixels))) / global_state.callback_os_window->fonts_data->cell_height;
+        if (pixels < 0) s *= -1;
+        global_state.callback_os_window->pending_scroll_pixels = pixels - s;
+    } else {
+        s = (int) round(yoffset * OPT(wheel_scroll_multiplier));
+        global_state.callback_os_window->pending_scroll_pixels = 0;
+    }
+    if (s == 0) return;
+    bool upwards = s > 0;
+    Screen *screen = w->render_data.screen;
+    if (screen->linebuf == screen->main_linebuf) {
+        screen_history_scroll(screen, abs(s), upwards);
+    } else {
+        if (screen->modes.mouse_tracking_mode) {
+            int sz = encode_mouse_event(w, upwards ? GLFW_MOUSE_BUTTON_4 : GLFW_MOUSE_BUTTON_5, PRESS, 0);
+            if (sz > 0) { mouse_event_buf[sz] = 0; write_escape_code_to_child(screen, CSI, mouse_event_buf); }
         } else {
-            if (screen->modes.mouse_tracking_mode) {
-                int sz = encode_mouse_event(w, upwards ? GLFW_MOUSE_BUTTON_4 : GLFW_MOUSE_BUTTON_5, PRESS, 0);
-                if (sz > 0) { mouse_event_buf[sz] = 0; write_escape_code_to_child(screen, CSI, mouse_event_buf); }
-            } else {
-                fake_scroll(abs(s), upwards);
-            }
+            fake_scroll(abs(s), upwards);
         }
     }
 }

kitty/state.h

@@ -124,6 +124,7 @@ typedef struct {
     float background_opacity;
     FONTS_DATA_HANDLE fonts_data;
     id_type temp_font_group_id;
+    double pending_scroll_pixels;
 } OSWindow;