Fix triple-click line selection not extending wrapped lines beyond viewport bottom

Similar to commit 625e984 which fixed extending into scrollback (above viewport),
this fix extends line selection below the viewport when a wrapped line continues
past the bottom edge. Adds continue_line_downwards_offscreen() and applies it
in both the initial selection and extending selection code paths.

Agent-Logs-Url: https://github.com/kovidgoyal/kitty/sessions/e548ce84-fdb7-4fd1-b3df-e1166b45f5bd

Co-authored-by: kovidgoyal <1308621+kovidgoyal@users.noreply.github.com>

docs/changelog.rst

@@ -217,6 +217,8 @@ Detailed list of changes
 
 - Fix dragging of splits layout borders sometimes moving in the wrong direction or having no effect (:pull:`9447`)
 
+- Fix triple-click line selection not extending wrapped lines beyond the bottom edge of the viewport
+
 - Password input in kittens: hide the cursor and display a blinking 🔒 at the end of typed characters to make it visually clear the user is entering a password
 
 - edit-in-kitty: Ignore environment variables as some editors execute code present in env vars

kitty/screen.c

@@ -5395,6 +5395,17 @@ continue_line_downwards(Screen *self, index_type bottom_line, SelectionBoundary
     return bottom_line;
 }
 
+static index_type
+continue_line_downwards_offscreen(Screen *self, int bottom_line, SelectionBoundary *start, SelectionBoundary *end) {
+    index_type num_offscreen = 0;
+    Line *line = NULL;
+    while ((line = checked_range_line(self, bottom_line + 1)) && range_line_is_continued(self, bottom_line + 1)) {
+        screen_selection_range_for_line_(line, &start->x, &end->x);
+        bottom_line++; num_offscreen++;
+    }
+    return num_offscreen;
+}
+
 static int
 clamp_selection_input_to_multicell(Screen *self, const Selection *s, index_type x, index_type y, bool in_left_half_of_cell) {
     int delta = 0;
@@ -5538,6 +5549,16 @@ do_update_selection(Screen *self, Selection *s, index_type x, index_type y, bool
                                 s->start.x = up_start.x;
                             }
                         }
+                        // extend below viewport if needed
+                        if (bottom_line >= self->lines - 1 && self->scrolled_by > 0 && self->linebuf == self->main_linebuf) {
+                            int range_bottom = (int)bottom_line - (int)self->scrolled_by;
+                            index_type num_below_viewport = continue_line_downwards_offscreen(
+                                    self, range_bottom, &down_start, &down_end);
+                            if (num_below_viewport) {
+                                s->end_scrolled_by -= num_below_viewport;
+                                s->end.x = down_end.x;
+                            }
+                        }
                     }
                 }
 #undef S
@@ -5562,6 +5583,16 @@ do_update_selection(Screen *self, Selection *s, index_type x, index_type y, bool
                         }
                     } else {
                         a->in_left_half_of_cell = false; a->x = down_end.x; a->y = bottom_line;
+                        // extend below viewport if needed
+                        if (bottom_line >= self->lines - 1 && self->scrolled_by > 0 && self->linebuf == self->main_linebuf) {
+                            int range_bottom = (int)bottom_line - (int)self->scrolled_by;
+                            index_type num_below_viewport = continue_line_downwards_offscreen(
+                                    self, range_bottom, &down_start, &down_end);
+                            if (num_below_viewport) {
+                                s->end_scrolled_by -= num_below_viewport;
+                                s->end.x = down_end.x;
+                            }
+                        }
                     }
                     // allow selecting whitespace at the start of the top line
                     if (a->y == top_line && s->input_current.y == top_line && s->input_current.x < a->x && adjusted_boundary_is_before) a->x = s->input_current.x;

kitty_tests/mouse.py

@@ -255,6 +255,33 @@ class TestMouse(BaseTest):
         release(x=2, button=GLFW_MOUSE_BUTTON_RIGHT)
         self.ae(sel(), 'ABCDE12345\n678')
 
+        # line select for wrapped lines below viewport
+        s.reset()
+        s.scroll(100, False)
+        # draw enough content to enable scrolling: fill lines-1 visual lines, then draw a long wrapped line
+        s.draw(('X' * s.columns) * (s.lines - 1))
+        s.linefeed(), s.carriage_return()
+        s.draw('ABCDE12345')  # wraps to 2 visual lines, second one pushed below viewport
+        # scroll up by 1 so top visual line of the wrapped text is visible but bottom wraps off-screen
+        s.scroll(1, True)
+        multi_click(x=1, y=s.lines - 1, count=3)
+        self.ae(sel(), 'ABCDE12345')
+        # extending selection to a line that wraps below viewport
+        s.reset()
+        s.scroll(100, False)
+        s.draw(('X' * s.columns) * (s.lines - 2))
+        s.linefeed(), s.carriage_return()
+        s.draw('678')
+        s.linefeed(), s.carriage_return()
+        s.draw('ABCDE12345')
+        s.scroll(1, True)
+        multi_click(x=1, y=3, count=3)
+        self.ae(sel(), '678')
+        press(x=2, y=s.lines - 1, button=GLFW_MOUSE_BUTTON_RIGHT)
+        release(x=2, y=s.lines - 1, button=GLFW_MOUSE_BUTTON_RIGHT)
+        self.ae(sel(), '678\nABCDE12345')
+        s.scroll(100, False)
+
         # Rectangle select
         init()
         press(x=1, y=1, modifiers=GLFW_MOD_ALT | GLFW_MOD_CONTROL)