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?

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;