fix(terminal): avoid memory corruption in `cursorScrollDown`

It was previously possible for `eraseRow` to move the cursor pin to a
different page, and then the call to `cursorChangePin` would try to free
the cursor style from that page even though that's not the page it
belongs to, which creates memory corruption in release modes and
integrity violations or assertions in debug mode.

As a bonus, this should actually be faster this way than the old code,
since it avoids needless work that `cursorChangePin` otherwise does.

src/terminal/Screen.zig

@@ -756,6 +756,11 @@ pub fn cursorDownScroll(self: *Screen) !void {
             var dirty = page.dirtyBitSet();
             dirty.set(0);
         } else {
+            // The call to `eraseRow` will move the tracked cursor pin up by one
+            // row, but we don't actually want that, so we keep the old pin and
+            // put it back after calling `eraseRow`.
+            const old_pin = self.cursor.page_pin.*;
+
             // eraseRow will shift everything below it up.
             try self.pages.eraseRow(.{ .active = .{} });
 
@@ -763,26 +768,15 @@ pub fn cursorDownScroll(self: *Screen) !void {
             // because eraseRow will mark all the rotated rows as dirty
             // in the entire page.
 
-            // We need to move our cursor down one because eraseRows will
+            // We don't use `cursorChangePin` here because we aren't
-            // preserve our pin directly and we're erasing one row.
+            // actually changing the pin, we're keeping it the same.
-            const page_pin = self.cursor.page_pin.down(1).?;
+            self.cursor.page_pin.* = old_pin;
-            self.cursorChangePin(page_pin);
+
-            const page_rac = page_pin.rowAndCell();
+            // We do, however, need to refresh the cached page row
+            // and cell, because `eraseRow` will have moved the row.
+            const page_rac = self.cursor.page_pin.rowAndCell();
             self.cursor.page_row = page_rac.row;
             self.cursor.page_cell = page_rac.cell;
-
-            // The above may clear our cursor so we need to update that
-            // again. If this fails (highly unlikely) we just reset
-            // the cursor.
-            self.manualStyleUpdate() catch |err| {
-                // This failure should not happen because manualStyleUpdate
-                // handles page splitting, overflow, and more. This should only
-                // happen if we're out of RAM. In this case, we'll just degrade
-                // gracefully back to the default style.
-                log.err("failed to update style on cursor scroll err={}", .{err});
-                self.cursor.style = .{};
-                self.cursor.style_id = 0;
-            };
         }
     } else {
         const old_pin = self.cursor.page_pin.*;