csi: erase scrollback: cancel selection if it touches the scrollback

This breaks out the scrollback erasing logic for \E[3J from csi.c, and
moves it to the new function term_erase_scrollback(), and changes the
logic to calculate the start and end row (absolute) numbers of the
scrollback, and only iterate those, instead of iterating *all* rows,
filtering out those that are on-screen.

It also adds an intersection range check of the selection range, and
cancels the selection if it touches any of the deleted scrollback
rows.

This fixes a crash when trying to render the next frame, since the
selection now references rows that have been freed.

Closes #633

csi.c

@@ -917,26 +917,7 @@ csi_dispatch(struct terminal *term, uint8_t final)
 
             case 3: {
                 /* Erase scrollback */
-                int end = (term->grid->offset + term->rows - 1) % term->grid->num_rows;
-                for (size_t i = 0; i < term->grid->num_rows; i++) {
-                    if (end >= term->grid->offset) {
-                        /* Not wrapped */
-                        if (i >= term->grid->offset && i <= end)
-                            continue;
-                    } else {
-                        /* Wrapped */
-                        if (i >= term->grid->offset || i <= end)
-                            continue;
-                    }
-
-                    if (term->render.last_cursor.row == term->grid->rows[i])
-                        term->render.last_cursor.row = NULL;
-
-                    grid_row_free(term->grid->rows[i]);
-                    term->grid->rows[i] = NULL;
-                }
-                term->grid->view = term->grid->offset;
-                term_damage_view(term);
+                term_erase_scrollback(term);
                 break;
             }
 

selection.c

@@ -69,10 +69,8 @@ selection_on_rows(const struct terminal *term, int row_start, int row_end)
         end = tmp;
     }
 
-    if (row_start >= start->row && row_end <= end->row) {
-        LOG_INFO("ON ROWS");
+    if (row_start >= start->row && row_end <= end->row)
         return true;
-    }
 
     return false;
 }

terminal.c

@@ -2011,6 +2011,69 @@ term_erase(struct terminal *term, const struct coord *start, const struct coord
     sixel_overwrite_by_row(term, end->row, 0, end->col + 1);
 }
 
+void
+term_erase_scrollback(struct terminal *term)
+{
+    const int mask = term->grid->num_rows - 1;
+    const int start = (term->grid->offset + term->rows) & mask;
+    const int end = (term->grid->offset - 1) & mask;
+    const int sel_start = term->selection.start.row;
+    const int sel_end = term->selection.end.row;
+
+    if (sel_end >= 0) {
+        /*
+         * Cancel selection if it touches any of the rows in the
+         * scrollback, since we can’t have the selection reference
+         * soon-to-be deleted rows.
+         *
+         * This is done by range checking the selection range against
+         * the scrollback range.
+         *
+         * To make this comparison simpler, the start/end absolute row
+         * numbers are “rebased” against the scrollback start, where
+         * row 0 is the *first* row in the scrollback. A high number
+         * thus means the row is further *down* in the scrollback,
+         * closer to the screen bottom.
+         */
+        int scrollback_start = term->grid->offset + term->rows;
+
+        int rel_sel_start = sel_start - scrollback_start + term->grid->num_rows;
+        int rel_sel_end = sel_end - scrollback_start + term->grid->num_rows;
+
+        int rel_start = start - scrollback_start + term->grid->num_rows;
+        int rel_end = end - scrollback_start + term->grid->num_rows;
+
+        rel_sel_start &= mask;
+        rel_sel_end &= mask;
+        rel_start &= mask;
+        rel_end &= mask;
+
+        if ((rel_sel_start <= rel_start && rel_sel_end >= rel_start) ||
+            (rel_sel_start <= rel_end && rel_sel_end >= rel_end) ||
+            (rel_sel_start >= rel_start && rel_sel_end <= rel_end))
+        {
+            selection_cancel(term);
+        }
+    }
+
+    for (int i = start;; i = (i + 1) & mask) {
+        struct row *row = term->grid->rows[i];
+        if (row != NULL) {
+            if (term->render.last_cursor.row == row)
+                term->render.last_cursor.row = NULL;
+
+            grid_row_free(row);
+            term->grid->rows[i] = NULL;
+        }
+
+        if (i == end)
+            break;
+    }
+
+    term->grid->view = term->grid->offset;
+    term_damage_view(term);
+}
+
 int
 term_row_rel_to_abs(const struct terminal *term, int row)
 {

terminal.h

@@ -662,6 +662,7 @@ void term_damage_scroll(
 
 void term_erase(
     struct terminal *term, const struct coord *start, const struct coord *end);
+void term_erase_scrollback(struct terminal *term);
 
 int term_row_rel_to_abs(const struct terminal *term, int row);
 void term_cursor_home(struct terminal *term);