don't treat programmatic selection changes as cursor movement in AceNodeView

[kevinushey]

Fixes #1099.

AceNodeView.setSelection() sets this.updating, focuses the Ace editor and applies a selection range. That fires changeCursor, and the handler set cursorDirty without consulting this.updating — so the next Ace render called scrollCursorIntoView() and scrolled the editing root to that node view, even though nothing moved the cursor. The value-change handlers just above (L508/L513) already guard on this.updating; this one didn't.

ProseMirror calls setSelection() from selectionToDOM() on every EditorView.focus(), so any focus restore scrolled the document to whichever embedded editor held the selection. For a freshly opened document that selection is at position 0 — the YAML front matter block — so the document snapped to the very top. In RStudio this shows up as the visual editor jumping to the top of the document whenever you run a code chunk in a file you have just opened (rstudio/rstudio#18490); clicking once anywhere in the prose moves the selection out of an AceNodeView and the symptom disappears until the file is reopened.

Verification

Tested against an RStudio development build by serving a patched panmirror.js to the browser and leaving everything else identical, measuring .pm-scroll-container scrollTop across a chunk run:

	run 1	run 2
before	jumps 215px to the top, then scrolls back	jumps 163px to the top, then scrolls back
after	no movement	no movement

Genuine cursor-driven scrollback is unchanged: scrolling a focused code chunk out of view and pressing an arrow key still brings the cursor back into view, identically before and after.

Notes

No apps/vscode/CHANGELOG.md entry — aceExtension is only installed when options.codeEditor === 'ace' (editor-extensions.ts:222), which is RStudio's configuration; the VS Code and Positron visual editors use the CodeMirror code view and are unaffected. Happy to add an entry if you'd rather have one.

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