# Buggy code
def paintEvent(self, event):
    """Override Qt method."""
    painter = QPainter(self)

    color = QColor(self.color)
    color.setAlphaF(0.5)
    painter.setPen(color)
    offset = self.editor.document().documentMargin() + self.editor.contentOffset().x()
    folding_panel = self.editor.panels.get("FoldingPanel")
    folding_regions = folding_panel.folding_regions
    folding_status = folding_panel.folding_status
    leading_whitespaces = self.editor.leading_whitespaces
    for line_number in folding_regions:
        post_update = False
        end_line = folding_regions[line_number]
        start_block = self.editor.document().findBlockByNumber(line_number)
        end_block = self.editor.document().findBlockByNumber(end_line - 1)
        top = int(
            self.editor.blockBoundingGeometry(start_block)
            .translated(self.editor.contentOffset())
            .top()
        )
        bottom = int(
            self.editor.blockBoundingGeometry(end_block)
            .translated(self.editor.contentOffset())
            .bottom()
        )
        total_whitespace = leading_whitespaces[max(line_number - 1, 0)]
        end_whitespace = leading_whitespaces[end_line - 1]
        if end_whitespace != total_whitespace:
            x = (
                self.editor.fontMetrics().width(total_whitespace * "9")
                + self.bar_offset
                + offset
            )
            painter.drawLine(x, top, x, bottom)


# Traceback (KeyError)
File "/home/carlos/Projects/spyder/github-repo/spyder/plugins/completion/plugin.py", line 120, in receive_response

Traceback (most recent call last):
File "/home/carlos/Projects/spyder/github-repo/spyder/plugins/completion/plugin.py", line 120, in receive_response
self._maybe_send_locked(req_id)
File "/home/carlos/Projects/spyder/github-repo/spyder/plugins/completion/plugin.py", line 161, in _maybe_send_locked
send()
File "/home/carlos/Projects/spyder/github-repo/spyder/plugins/completion/plugin.py", line 143, in send
self.gather_and_send(request_responses)
File "/home/carlos/Projects/spyder/github-repo/spyder/plugins/completion/plugin.py", line 212, in gather_and_send
response_instance.handle_response(req_type, responses)
File "/home/carlos/Projects/spyder/github-repo/spyder/plugins/editor/widgets/codeeditor.py", line 928, in handle_response
handler(params)
File "/home/carlos/Projects/spyder/github-repo/spyder/plugins/editor/widgets/codeeditor.py", line 1325, in handle_folding_range
folding_panel.update_folding(ranges)
File "/home/carlos/Projects/spyder/github-repo/spyder/plugins/editor/panels/codefolding.py", line 220, in update_folding
folding_status[new_line] = self.folding_status[line]
KeyError: 297

# Fixed code
def paintEvent(self, event):
    """Override Qt method."""
    painter = QPainter(self)

    color = QColor(self.color)
    color.setAlphaF(0.5)
    painter.setPen(color)
    offset = self.editor.document().documentMargin() + self.editor.contentOffset().x()
    folding_panel = self.editor.panels.get("FoldingPanel")
    folding_regions = folding_panel.folding_regions
    folding_status = folding_panel.folding_status
    leading_whitespaces = self.editor.leading_whitespaces
    for line_number in folding_regions:
        post_update = False
        end_line = folding_regions[line_number]
        start_block = self.editor.document().findBlockByNumber(line_number)
        end_block = self.editor.document().findBlockByNumber(end_line - 1)
        top = int(
            self.editor.blockBoundingGeometry(start_block)
            .translated(self.editor.contentOffset())
            .top()
        )
        bottom = int(
            self.editor.blockBoundingGeometry(end_block)
            .translated(self.editor.contentOffset())
            .bottom()
        )
        total_whitespace = leading_whitespaces.get(max(line_number - 1, 0))
        end_whitespace = leading_whitespaces.get(end_line - 1)
        if end_whitespace and end_whitespace != total_whitespace:
            x = (
                self.editor.fontMetrics().width(total_whitespace * "9")
                + self.bar_offset
                + offset
            )
            painter.drawLine(x, top, x, bottom)