Sync to monitor refreshes by default

Prevents tearing while scrolling. See #318

Reduce draw calls while processing mouse move events to make latency
while selecting with the mouse acceptable. Also add a config option to
turn off monitor syncing, so people with keyboard repeat rates higher
than their monitor's refresh rate can use it.

CHANGELOG.rst

@@ -25,8 +25,8 @@ version 0.8.0 [future]
   variation presentation selector.
 
 - Prevent video tearing during high speed scrolling by syncing draws
-  to the monitor's refresh rate (only works if supported by the graphics
-  drivers).
+  to the monitor's refresh rate. There is a new configuration option to
+  control this ``sync_to_monitor``.
 
 - Add some non standard terminfo capabilities used by neovim and tmux.
 

README.asciidoc

@@ -351,6 +351,8 @@ smoothness.
 There are two parameters you can tune to adjust the performance. ``repaint_delay``
 and ``input_delay``. These control the artificial delays introduced into the
 render loop to reduce CPU usage. See the link:kitty/kitty.conf[config file] for details.
+See also the ``sync_to_monitor`` option to further decrease latency at the cost
+of some link:https://en.wikipedia.org/wiki/Screen_tearing[tearing] while scrolling.
 
 You can generate detailed per-function performance data using
 link:https://github.com/gperftools/gperftools[gperftools]. Build kitty with

kitty/child-monitor.c

@@ -591,6 +591,7 @@ prepare_to_render_os_window(OSWindow *os_window, double now, unsigned int *activ
             if (is_active_window) {
                 *active_window_id = w->id;
                 collect_cursor_info(&WD.screen->cursor_render_info, w, now, os_window);
+                if (w->cursor_visible_at_last_render != WD.screen->cursor_render_info.is_visible) needs_render = true;
                 update_window_title(w, os_window);
             } else WD.screen->cursor_render_info.is_visible = false;
             if (send_cell_data_to_gpu(WD.vao_idx, WD.gvao_idx, WD.xstart, WD.ystart, WD.dx, WD.dy, WD.screen, os_window)) needs_render = true;
@@ -618,6 +619,7 @@ render_os_window(OSWindow *os_window, double now, unsigned int active_window_id)
                 double bell_left = global_state.opts.visual_bell_duration - (now - WD.screen->start_visual_bell_at);
                 set_maximum_wait(bell_left);
             }
+            w->cursor_visible_at_last_render = WD.screen->cursor_render_info.is_visible;
         }
     }
     swap_window_buffers(os_window);
@@ -640,7 +642,7 @@ render(double now) {
     for (size_t i = 0; i < global_state.num_os_windows; i++) {
         OSWindow *w = global_state.os_windows + i;
         if (!w->num_tabs || !should_os_window_be_rendered(w)) continue;
-        bool needs_render = w->is_focused || w->is_damaged;
+        bool needs_render = w->is_damaged;
         make_os_window_context_current(w);
         if (w->viewport_size_dirty) {
             w->clear_count = 0;

kitty/config.py

@@ -271,6 +271,7 @@ def url_style(x):
     'rectangle_select_modifiers': to_modifiers,
     'repaint_delay': positive_int,
     'input_delay': positive_int,
+    'sync_to_monitor': to_bool,
     'window_border_width': positive_float,
     'window_margin_width': positive_float,
     'window_padding_width': positive_float,

kitty/glfw.c

@@ -346,7 +346,7 @@ create_os_window(PyObject UNUSED *self, PyObject *args) {
     glfwMakeContextCurrent(glfw_window);
     if (x != -1 && y != -1) glfwSetWindowPos(glfw_window, x, y);
     current_os_window_ctx = glfw_window;
-    glfwSwapInterval(0);  // a value of 1 makes mouse selection laggy
+    glfwSwapInterval(OPT(sync_to_monitor) ? 1 : 0);  // a value of 1 makes mouse selection laggy
     if (is_first_window) {
         set_dpi_from_os_window(NULL);
         gl_init();

kitty/kitty.conf

@@ -127,7 +127,9 @@ initial_window_height  400
 
 # Delay (in milliseconds) between screen updates. Decreasing it, increases
 # frames-per-second (FPS) at the cost of more CPU usage. The default value
-# yields ~100 FPS which is more than sufficient for most uses.
+# yields ~100 FPS which is more than sufficient for most uses. Note that to
+# actually achieve 100FPS you have to either set sync_to_monitor to no or use a
+# monitor with a high refresh rate.
 repaint_delay    10
 
 # Delay (in milliseconds) before input from the program running in the terminal
@@ -137,6 +139,13 @@ repaint_delay    10
 # screen updates will be drawn.
 input_delay 3
 
+# Sync screen updates to the refresh rate of the monitor. This prevents
+# tearing (https://en.wikipedia.org/wiki/Screen_tearing) when scrolling. However,
+# it limits the rendering speed to the refresh rate of your monitor. With a
+# very high speed mouse/high keyboard repeat rate, you may notice some slight input latency.
+# If so, set this to no.
+sync_to_monitor yes
+
 # Visual bell duration. Flash the screen when a bell occurs for the specified number of
 # seconds. Set to zero to disable.
 visual_bell_duration 0.0

kitty/state.c

@@ -349,6 +349,7 @@ PYWRAP1(set_options) {
     S(background, color_as_int);
     S(repaint_delay, repaint_delay);
     S(input_delay, repaint_delay);
+    S(sync_to_monitor, PyObject_IsTrue);
     S(macos_option_as_alt, PyObject_IsTrue);
     S(macos_hide_titlebar, PyObject_IsTrue);
 

kitty/state.h

@@ -30,6 +30,7 @@ typedef struct {
     float background_opacity;
     float inactive_text_alpha;
     Edge tab_bar_edge;
+    bool sync_to_monitor;
 } Options;
 
 typedef struct {
@@ -55,7 +56,7 @@ typedef struct {
 
 typedef struct {
     id_type id;
-    bool visible;
+    bool visible, cursor_visible_at_last_render;
     PyObject *title;
     ScreenRenderData render_data;
     unsigned int mouse_cell_x, mouse_cell_y;
@@ -108,10 +109,7 @@ typedef struct {
     bool is_key_pressed[MAX_KEY_COUNT];
     bool viewport_size_dirty;
     double last_resize_event_at;
-    bool has_pending_resizes;
-    bool is_semi_transparent;
-    bool shown_once;
-    bool is_damaged;
+    bool has_pending_resizes, is_semi_transparent, shown_once, is_damaged;
     uint32_t offscreen_texture_id;
     unsigned int clear_count;
 } OSWindow;