findAlignedLines sometimes returns unexpected results

[HaudinFlorence]

In some case it has been observed that the method findAlignedLinesused to determine which line numbers align with each other, in the set of DiffViews, return a non correct result, leading to wrong alignment.

We can see here an example of a mergeview:

The result given by findAlignedLinesis.
[3, 3, 3, 3]
[4, 5, 4, 5]
[11, 11, 11, 12]
[18, 18, 13, 14]
There is an additionnal padding in the left editor above line 12 causing that this line is not aligned with the 2 lines 11 of central and right editors.

After proposing this manual correction for the array return by findAlignedLines, alignment is fixed:
[3, 3, 3, 3]
[4, 5, 4, 5]
[11, 12 11, 12]
[18, 19, 13, 14]

[fcollonval]

That error is more specifically coming from

nbdime/packages/nbdime/src/common/mergeview.ts

Line 997 in 8eb311d

function getMatchingEditLineLC(toMatch: Chunk, chunks: Chunk[]): number {

it does not handle properly finding a chunk in an editor that does not have that chunk; e.g. if a chunk is between base and right editors, that method does not return the proper alignment for that chunk in the left editor.