Breakpoints are not hit when cell source begins with empty lines

[kycutler]

Description

When connected to a debuggable kernel (this reproduces with both latest ipykernel and xpython), breakpoints in a cell that begins with one or more "empty" lines (whitespace only) will never be hit. Removing the lines and re-running the cell fixes the issue.

Reproduce

1. Create a new notebook
2. Connect to a debuggable kernel and enable the debugger
3. Add one or more empty lines to the cell, followed by some code (e.g. print("hello world")) and set a breakpoint on the line with code
4. Run the cell
5. Notice that execution does not pause on the breakpoint

Expected behavior

The breakpoint should be hit regardless of leading newlines

Context

• Operating System and version: Windows 11
• Browser and version: Edge 95
• JupyterLab version: 3.2.4

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[kycutler]

Testing this a bit more, it seems the initial empty lines are not counted toward line numbers in the debugger. Breakpoints can still be hit if they are on a line number that is less than the number of lines after the initial empty ones. For example with three non-empty lines they can be hit up to line three:

Notice also that the outputs indicate that we are actually on print(3) since 1 and 2 have been printed already. So it seems just the line numbers are wrong.

[fcollonval]

The reason for this bug is the cleanup transformation applied by IPython:

https://github.com/ipython/ipython/blob/c5102802380809adfdb980dafd041d99c5c9e299/IPython/core/inputtransformer2.py#L571

One of those cleanup transformation removes the leading empty lines. I pushed a fix to ipykernel that deactivates that cleanup transformation when a debug session is active (PR link above this comment).