# Buggy code
def paintEvent(self, event):
    # Paints the fold indicators and the possible fold region background
    # on the folding panel.
    super(FoldingPanel, self).paintEvent(event)
    painter = QPainter(self)
    if not self._display_folding and not self._key_pressed:
        if any(self.folding_status.values()):
            for info in self.editor.visible_blocks:
                top_position, line_number, block = info
                self._draw_collapsed_indicator(
                    line_number, top_position, block, painter, mouse_hover=True
                )
        return
    # Draw background over the selected non collapsed fold region
    if self._mouse_over_line is not None:
        block = self.editor.document().findBlockByNumber(self._mouse_over_line)
        try:
            self._draw_fold_region_background(block, painter)
        except (ValueError, KeyError):
            # Catching the KeyError above is necessary to avoid
            # issue spyder-ide/spyder#10918.
            # It happens when users have the mouse on top of the
            # folding panel and make some text modifications
            # that trigger a folding recomputation.
            pass
    # Draw fold triggers
    for top_position, line_number, block in self.editor.visible_blocks:
        self._draw_collapsed_indicator(
            line_number, top_position, block, painter, mouse_hover=False
        )


# Traceback (KeyError)
File "C:\Users\marco\Anaconda3\lib\site-packages\spyder\plugins\editor\utils\editor.py", line 161, in _exec_requested_job
self._job(*self._args, **self._kwargs)
File "C:\Users\marco\Anaconda3\lib\site-packages\spyder\plugins\editor\panels\codefolding.py", line 471, in _highlight_block
end_line = self.folding_regions[block_line]
KeyError: 155

# Fixed code
def paintEvent(self, event):
    # Paints the fold indicators and the possible fold region background
    # on the folding panel.
    super(FoldingPanel, self).paintEvent(event)
    painter = QPainter(self)
    document = self.editor.document()

    if not self._display_folding and not self._key_pressed:
        if any(self.folding_status.values()):
            for info in self.editor.visible_blocks:
                top_position, line_number, block = info
                self._draw_collapsed_indicator(
                    line_number, top_position, block, painter, mouse_hover=True
                )
        return
    # Draw background over the selected non collapsed fold region
    if self._mouse_over_line is not None:
        block = self.editor.document().findBlockByNumber(self._mouse_over_line)
        try:
            self._draw_fold_region_background(block, painter)
        except (ValueError, KeyError):
            # Catching the KeyError above is necessary to avoid
            # issue spyder-ide/spyder#10918.
            # It happens when users have the mouse on top of the
            # folding panel and make some text modifications
            # that trigger a folding recomputation.
            pass
    # Draw fold triggers
    for top_position, line_number, block in self.editor.visible_blocks:
        self._draw_collapsed_indicator(
            line_number, top_position, block, painter, mouse_hover=False
        )