Implement a simple scroll progress indicator

Shows a simple bar on the right edge of the window that moves up as you
scroll further back. There are apparently a lot of people that dont use
a pager for browsing large scrollbacks. I will never understand this,
but, what the hell I was in that code area anyway for other reasons.

TODO: Maybe make it a rounded rectangle

docs/changelog.rst

@@ -54,6 +54,8 @@ Detailed list of changes
   using the panel kitten for all compositors that support the requisite Wayland
   protocol which is practically speaking all of them but GNOME (:pull:`2590`)
 
+- A new option :opt:`scrollback_indicator_opacity` to show a small indicator on the right hand side of the screen of how much you have scrolled
+
 - Wayland: Support fractional scales so that there is no wasted drawing at larger scale followed by resizing in the compositor
 
 - Wayland: Support preferred integer scales

kitty/options/definition.py

@@ -374,6 +374,14 @@ is changed it will only affect newly created windows, not existing ones.
 '''
     )
 
+opt('scrollback_indicator_opacity', '0.0',
+    option_type='unit_float', ctype='float', long_text='''
+The opacity of the scrollback indicator which is a small colored rectangle that moves
+along the right hand side of the window as you scroll, indicating what fraction you
+have scrolled. The default is zero which means fully transparent, aka invisible.
+Set to a value between zero and one to see the indicator.''')
+
+
 opt('scrollback_pager', 'less --chop-long-lines --RAW-CONTROL-CHARS +INPUT_LINE_NUMBER',
     option_type='to_cmdline',
     long_text='''

kitty/options/parse.py

@@ -1172,6 +1172,9 @@ class Parser:
     def scrollback_fill_enlarged_window(self, val: str, ans: typing.Dict[str, typing.Any]) -> None:
         ans['scrollback_fill_enlarged_window'] = to_bool(val)
 
+    def scrollback_indicator_opacity(self, val: str, ans: typing.Dict[str, typing.Any]) -> None:
+        ans['scrollback_indicator_opacity'] = unit_float(val)
+
     def scrollback_lines(self, val: str, ans: typing.Dict[str, typing.Any]) -> None:
         ans['scrollback_lines'] = scrollback_lines(val)
 

kitty/options/to-c-generated.h

@@ -135,6 +135,19 @@ convert_from_opts_cursor_stop_blinking_after(PyObject *py_opts, Options *opts) {
     Py_DECREF(ret);
 }
 
+static void
+convert_from_python_scrollback_indicator_opacity(PyObject *val, Options *opts) {
+    opts->scrollback_indicator_opacity = PyFloat_AsFloat(val);
+}
+
+static void
+convert_from_opts_scrollback_indicator_opacity(PyObject *py_opts, Options *opts) {
+    PyObject *ret = PyObject_GetAttrString(py_opts, "scrollback_indicator_opacity");
+    if (ret == NULL) return;
+    convert_from_python_scrollback_indicator_opacity(ret, opts);
+    Py_DECREF(ret);
+}
+
 static void
 convert_from_python_scrollback_pager_history_size(PyObject *val, Options *opts) {
     opts->scrollback_pager_history_size = PyLong_AsUnsignedLong(val);
@@ -1132,6 +1145,8 @@ convert_opts_from_python_opts(PyObject *py_opts, Options *opts) {
     if (PyErr_Occurred()) return false;
     convert_from_opts_cursor_stop_blinking_after(py_opts, opts);
     if (PyErr_Occurred()) return false;
+    convert_from_opts_scrollback_indicator_opacity(py_opts, opts);
+    if (PyErr_Occurred()) return false;
     convert_from_opts_scrollback_pager_history_size(py_opts, opts);
     if (PyErr_Occurred()) return false;
     convert_from_opts_scrollback_fill_enlarged_window(py_opts, opts);

kitty/options/types.py

@@ -402,6 +402,7 @@ option_names = (  # {{{
  'resize_debounce_time',
  'resize_in_steps',
  'scrollback_fill_enlarged_window',
+ 'scrollback_indicator_opacity',
  'scrollback_lines',
  'scrollback_pager',
  'scrollback_pager_history_size',
@@ -562,6 +563,7 @@ class Options:
     resize_debounce_time: typing.Tuple[float, float] = (0.1, 0.5)
     resize_in_steps: bool = False
     scrollback_fill_enlarged_window: bool = False
+    scrollback_indicator_opacity: float = 0
     scrollback_lines: int = 2000
     scrollback_pager: typing.List[str] = ['less', '--chop-long-lines', '--RAW-CONTROL-CHARS', '+INPUT_LINE_NUMBER']
     scrollback_pager_history_size: int = 0

kitty/shaders.c

@@ -594,6 +594,31 @@ draw_tint(bool premult, Screen *screen, const CellRenderData *crd) {
     glDrawArrays(GL_TRIANGLE_FAN, 0, 4);
 }
 
+static bool
+draw_scroll_indicator(bool premult, Screen *screen, const CellRenderData *crd) {
+    if (OPT(scrollback_indicator_opacity) <= 0 || screen->linebuf != screen->main_linebuf || !screen->scrolled_by) return false;
+    glEnable(GL_BLEND);
+    if (premult) { BLEND_PREMULT } else { BLEND_ONTO_OPAQUE }
+    bind_program(TINT_PROGRAM);
+    const color_type bar_color = colorprofile_to_color(screen->color_profile, screen->color_profile->overridden.highlight_bg, screen->color_profile->configured.highlight_bg).rgb;
+    GLfloat alpha = 0.8f;
+    float frac = (float)screen->scrolled_by / (float)screen->historybuf->count;
+    const GLfloat bar_height = crd->gl.dy;
+    GLfloat bottom = (crd->gl.ystart - crd->gl.height);
+    bottom += MAX(0, crd->gl.height - bar_height) * frac;
+#define C(shift) srgb_color((bar_color >> shift) & 0xFF) * premult_factor
+    GLfloat premult_factor = premult ? alpha : 1.0f;
+    glUniform4f(tint_program_layout.uniforms.tint_color, C(16), C(8), C(0), alpha);
+#undef C
+    GLfloat width = 0.5f * crd->gl.dx;
+    GLfloat left = (GLfloat)(crd->gl.xstart + (screen->columns * crd->gl.dx - width));
+    glUniform4f(tint_program_layout.uniforms.edges, left, bottom, left + width, bottom + bar_height);
+    glDrawArrays(GL_TRIANGLE_FAN, 0, 4);
+    glDisable(GL_BLEND);
+    return true;
+}
+
+
 static float prev_inactive_text_alpha = -1;
 
 static void
@@ -972,14 +997,16 @@ draw_cells(ssize_t vao_idx, const WindowRenderData *srd, OSWindow *os_window, bo
                 scale_rendered_graphic(grman->render_data.item + i, srd->xstart, srd->ystart, crd.x_ratio, crd.y_ratio);
         }
     }
+    bool use_premult = false;
     has_underlying_image |= grman->num_of_below_refs > 0 || grman->num_of_negative_refs > 0;
     if (os_window->is_semi_transparent) {
-        if (has_underlying_image) draw_cells_interleaved_premult(vao_idx, screen, os_window, &crd, wl);
+        if (has_underlying_image) { draw_cells_interleaved_premult(vao_idx, screen, os_window, &crd, wl); use_premult = true; }
         else draw_cells_simple(vao_idx, screen, &crd, os_window->is_semi_transparent);
     } else {
         if (has_underlying_image) draw_cells_interleaved(vao_idx, screen, os_window, &crd, wl);
         else draw_cells_simple(vao_idx, screen, &crd, os_window->is_semi_transparent);
     }
+    draw_scroll_indicator(use_premult, screen, &crd);
 
     if (screen->start_visual_bell_at) {
         GLfloat intensity = get_visual_bell_intensity(screen);

kitty/state.h

@@ -53,7 +53,7 @@ typedef struct {
     float macos_thicken_font;
     WindowTitleIn macos_show_window_title_in;
     char *bell_path, *bell_theme;
-    float background_opacity, dim_opacity;
+    float background_opacity, dim_opacity, scrollback_indicator_opacity;
     float text_contrast, text_gamma_adjustment;
     bool text_old_gamma;