`diffEditorViewModel` breaks when previous `_unchangedRegions` is non-empty

[OfekShilon]

The direct symptom is in the site www.compiler-explorer.com, but the investigation went through monaco-editor (which the site uses) and ends up here.

Steps to Reproduce:

1. Navigate to https://godbolt.org/z/dsav96z6P
   This shows the "LLVM Opt Pipeline Viewer" (1) which is a diff pane created by monaco's createDiffEditor (2). It's contents change based on the user selection in from the 'passes' list (3).
   

2. To see normal operation in action: click in order the passes "SROAPass" (first one) and "Greedy Register Allocator". Diffs are shown properly:
   

3. To reproduce the bug, click in order the passes "SROAPass", then "Debug Variable Analysis" (one above Greedy) and finally "Greedy Register Allocator". First, text diffs are not being decorated despite being present:

and second, a console shows the error:

lineRange.js:94  Uncaught (in promise) TypeError: Cannot read properties of undefined (reading 'endLineNumberExclusive')
    at nl.intersectsStrict (lineRange.js:94:45)
    at a (diffEditorViewModel.js:98:50)
    at diffEditorViewModel.js:180:17
    at BV (base.js:76:9)
    at BU.<anonymous> (diffEditorViewModel.js:179:13)
    at Generator.next (<anonymous>)
    at r (diffEditorViewModel.js:17:58)

---

Preliminary investigation directs attention to these lines in DiffEditorViewModel's updateUnchangedRegions:

        const updateUnchangedRegions = (result, tx, reader) => {
            const newUnchangedRegions = UnchangedRegion.fromDiffs(result.changes, model.original.getLineCount(), model.modified.getLineCount(), this._options.hideUnchangedRegionsMinimumLineCount.read(reader), this._options.hideUnchangedRegionsContextLineCount.read(reader));
            // Transfer state from cur state
            const lastUnchangedRegions = this._unchangedRegions.get();
            const lastUnchangedRegionsOrigRanges = lastUnchangedRegions.originalDecorationIds
                .map(id => model.original.getDecorationRange(id))
                .filter(r => !!r)
                .map(r => LineRange.fromRange(r));

...
            for (const r of newUnchangedRegions) {   
                for (let i = 0; i < lastUnchangedRegions.regions.length; i++) {
                    if (r.originalUnchangedRange.intersectsStrict(lastUnchangedRegionsOrigRanges[i])   <--- attempt to access an empty array
...
                    }
                }
            }

In the (not so rare) problematic scenario, newUnchangedRegions and lastUnchangedRegions.regions are non-empty but lastUnchangedRegionsOrigRanges is, so the code enters the loop and crashes immediately.

I suspect something broke in @hediet PR #191977, but not sure.

[OfekShilon]

A monaco-playground link with a small standalone repro for the problem:
https://microsoft.github.io/monaco-editor/playground.html?source=v0.44.0#XQAAAAJoAgAAAAAAAABBqQkHQ5NjdMjwa-jY7SIQ9S7DNlzs5W-mwj0fe1ZCDRFc9ws9XQE0SJE1jc2VKxhaLFIw9vEWSxW3yscw10ok-_1uhrp2RzO_KSoj3BnqSCFzt4AfU0xJ0CGYU1vRtgnKPmZzQcQ68ILtjJsSfOJtidapac8gxQKIxncegpZ2ULdDxvd_XPSiNVCchDoIge5jk6s1SWlJHGtnWQqr7L4r_MlNAJ2RzOsdk0zGe0j8TXj8yOvGPVTPTJTA8f3tvAZ_TMpaHmeOJ3xTggSlAUS-56fkH9W5H5tzIOm2S1a7ykHN4qD951XsRmPfdrrXpcQTePkwrf8b_5AwyLMoCEvdMdL8mF9vOFA_SC_blJZQOiu4Ou3FLGhIPmRj-zbDdoAdqYSMrjX9H30Rr_V5d4G_6RootmoxSUSxnj_-hnZM

This creates a diffView and populates both sides with an offending text (specifically, one with non-empty "unchangedRegions"). After two seconds it sets the left pane to the same text as it currently has, but this time the decorations disappear and a console error message as above appears.

Perhaps this tidbit would be helpful:

It seems that upon entering updateUnchangedRegions, this._unchangedRegions._value._originalDecorationIds contains ids that were already invalidated - i.e., do not appear in this.model.original._decorations:

And so .map(id => model.*.getDecorationRange(id)) returns only nulls and the resulting array is empty. My current guess is these decoration-IDs invalidation is too early, and should be moved to after updateUnchangedRegions.