Cursor moves position after deleting a character and changing line

Issue Report Checklist

• Searched the issues page for similar reports
• Read the relevant sections of the Spyder Troubleshooting Guide and followed its advice
• Reproduced the issue after updating with conda update spyder (or pip, if not using Anaconda)
• Could not reproduce inside jupyter qtconsole (if console-related)
• Tried basic troubleshooting (if a bug/error)
  • Restarted Spyder
  • Reset preferences with spyder --reset
  • Reinstalled the latest version of Anaconda
  • Tried the other applicable steps from the Troubleshooting Guide
• Completed the Problem Description, Steps to Reproduce and Version sections below

Problem Description

When replacing a simple string in selected text with Replace selection, Spyder takes a long time to replace it and to return control to the user.

What steps reproduce the problem?

1. Open a new Python file (or use temp.py or any existing Python file)
2. Write three lines with # Spyder is great (each in its own line)
3. Move the cursor in the first line in front of great
4. Press Del to delete the character g from the word great
5. Press the key Cursor down once

What is the expected output? What do you see instead?

I would expect that the cursor stays in the position before the character g in the next line, but it moves between g and r.

If I delete the character g with BACKSPACE, and move down one line, the cursor stays in the position between g and r, as expected.

Versions

• Spyder version: 4.1.2
• Python version: 3.6.9. 64-bit
• Qt version: 5.12.7
• PyQt version: 5.12.3
• Operating System name/version: Linux Mint 19.3 Cinnamon

Dependencies

# Mandatory:
atomicwrites >=1.2.0           :  1.3.0 (OK)
chardet >=2.0.0                :  3.0.4 (OK)
cloudpickle >=0.5.0            :  1.3.0 (OK)
diff_match_patch >=20181111    :  20181111 (OK)
intervaltree                   :  None (OK)
IPython >=4.0                  :  7.13.0 (OK)
jedi =0.15.2                   :  0.15.2 (OK)
keyring                        :  None (OK)
nbconvert >=4.0                :  5.6.1 (OK)
numpydoc >=0.6.0               :  0.9.2 (OK)
parso =0.5.2                   :  0.5.2 (OK)
pexpect >=4.4.0                :  4.8.0 (OK)
pickleshare >=0.4              :  0.7.5 (OK)
psutil >=5.3                   :  5.7.0 (OK)
pygments >=2.0                 :  2.6.1 (OK)
pylint >=0.25                  :  2.4.4 (OK)
pyls >=0.31.9;<0.32.0          :  0.31.9 (OK)
qdarkstyle >=2.8               :  2.8 (OK)
qtawesome >=0.5.7              :  0.7.0 (OK)
qtconsole >=4.6.0              :  4.7.1 (OK)
qtpy >=1.5.0                   :  1.9.0 (OK)
sphinx >=0.6.6                 :  2.4.4 (OK)
spyder_kernels >=1.9.0;<1.10.0 :  1.9.0 (OK)
watchdog                       :  None (OK)
xdg >=0.26                     :  0.26 (OK)
zmq >=17                       :  19.0.0 (OK)

# Optional:
cython >=0.21                  :  None (OK)
matplotlib >=2.0.0             :  None (OK)
numpy >=1.7                    :  1.18.2 (OK)
pandas >=0.13.1                :  None (OK)
scipy >=0.17.0                 :  1.4.1 (OK)
sympy >=0.7.3                  :  None (OK)

[hengin]

Indeed, this is due to how the QPlainTextEdit widget behaves. Here’s a small PyQt5 application where the behaviour can be reproduced:

from PyQt5.QtWidgets import QApplication, QPlainTextEdit
app = QApplication([])
widget = QPlainTextEdit()
widget.show()
app.exec_()

However, the Spyder CodeEditor widget already overrides keyboard input in https://github.com/spyder-ide/spyder/blob/9c84cd89625d9ad07f54810588ae6464fa4efef1/spyder/plugins/editor/widgets/codeeditor.py#L4254 so fixing this is easy. It suffices to replace the lines https://github.com/spyder-ide/spyder/blob/9c84cd89625d9ad07f54810588ae6464fa4efef1/spyder/plugins/editor/widgets/codeeditor.py#L4365 and https://github.com/spyder-ide/spyder/blob/9c84cd89625d9ad07f54810588ae6464fa4efef1/spyder/plugins/editor/widgets/codeeditor.py#L4386 by self.stdkey_backspace(): https://github.com/spyder-ide/spyder/blob/9c84cd89625d9ad07f54810588ae6464fa4efef1/spyder/plugins/editor/widgets/base.py#L933-L937

However, Spyder also has a similarly annoying behaviour with the “Delete” key that is not reproduced by the PyQt snippet above. I’ll try to track down the reason for that before making a PR. But a similar workaround should be possible.

[hengin]

For some reason (platform compatibility?), the Delete-key is handled as a shortcut. The problem disappears if I disable the “delete” shortcut in Tools > Preferences > Keyboard shortcuts.

The responsible piece of code is: https://github.com/spyder-ide/spyder/blob/9c84cd89625d9ad07f54810588ae6464fa4efef1/spyder/plugins/editor/widgets/codeeditor.py#L2218-L2227

Apparently QPlainTextEdit handles Delete events differently than the above. Indeed, if I manually perform the steps above in a QPlainTextEdit I can reproduce the behaviour. The recording below shows the behaviour of three different methods of character deletion in a QPlainTextEdit (same snippet as I posted above). In each case I press up/down immediately after pressing backspace/delete.

Actually, much larger jumps can be produced:

The reason is most likely that the QPlainTextEdit remembers the rightmost position the cursor was at just before the text is deleted and wants to place the cursor there again. So we just need to clear this memory. The simplest method I’ve found so far is

cursor = self.textCursor()
cursor.setPosition(cursor.position())
self.setTextCursor(cursor)

I’ll go ahead and make a PR now.

EDIT: The backspace issue (which likely has the same underlying cause as the delete issue) was reported as QTBUG-35861 seven years ago.