Improve rstudioapi::setSelectionRanges() for multi-byte characters

[juliasilge]

Related to #2582

Originally posted by @jennybc in #2582 (comment):

I think we probably have some more work to do here, around text that includes Unicode characters in the ✨astral plane"✨. This includes emoji, but also other stuff.

Here's my manual test:

1. Make a selection in the source editor.
2. In the Console, via typing / pasting/ or using the up-arrow, execute: x <- rstudioapi::getSourceEditorContext() to capture the current context, including selection data.
3. Now use rstudioapi::setSelectionRanges(x$selection[[1]][["range"]]) to complete the round trip, i.e. re-select the same selection.

We don't pass this test right now if, for example, the selection contains a emoji. RStudio does pass this test (although some of the numbers you see for positions are different). I think we want this sort of self-consistency, i.e. that we handle positions symmetrically for reading and setting.

My hunch is that PR needs to gain this sort of maneuver, but going the other way:

positron/src/vs/workbench/api/common/positron/extHostMethods.ts

Lines 137 to 168 in 7a3d14b

	// The selections in this text editor. The primary selection is always at index 0.
	//
	// The gymnastics here are so that we return character positions with respect to
	// Unicode code points. Otherwise, the native Position type provides offsets with respect to
	// UTF-16 encoded text. That would be confusing for downstream consumers, who probably
	// ultimately receive this text as UTF-8 and want to operate on this text in terms of
	// as user-perceivable "characters". This only matters when the selection's neighborhood
	// includes Unicode characters in the astral plane.
	//
	// Another resource that supports that what I'm doing here is desirable in Jupyter-land:
	// https://jupyter-client.readthedocs.io/en/latest/messaging.html#notes
	const selections = editor.selections.map(selection => {
	const lineTextBeforeActive = editor.document
	.lineAt(selection.active.line)
	.text.substring(0, selection.active.character);
	const unicodePointsBeforeActive = Array.from(lineTextBeforeActive).length;
	const lineTextBeforeStart = editor.document
	.lineAt(selection.start.line)
	.text.substring(0, selection.start.character);
	const unicodePointsBeforeStart = Array.from(lineTextBeforeStart).length;
	const text = editor.document.getText(selection);
	const unicodePointsInSelection = Array.from(text).length;
	return {
	active: { line: selection.active.line, character: unicodePointsBeforeActive },
	start: { line: selection.start.line, character: unicodePointsBeforeStart },
	end: { line: selection.end.line, character: unicodePointsBeforeStart + unicodePointsInSelection },
	text: text
	};
	});

---

Here's the basis of my manual test:

"🌷 b"

x <- rstudioapi::getSourceEditorContext()
f(x$selection[[1]])

rg <- x$selection[[1]][["range"]]
# creating the same document_range explicitly
# rg <- rstudioapi::document_range(
#   rstudioapi::document_position(1, 2),
#   rstudioapi::document_position(1, 5)
# )
rstudioapi::setSelectionRanges(rg)

# manual test cases
# select the text inside the quotes
# "a b"    round trip works
# "ä b"    round trip works
# "🌷 b"   round trip does NOT work, 'b' not part of selection
# "🤷‍♀️ b"   round trip does NOT work, 'b' not part of selection, but
#          for reasons documented elsewhere, we believe this is currently
#          out of scope

f <- function(sel) {
  cli::cli_inform("text: {.q {sel$text}}")
  cli::cli_inform("nchar(text): {.q {nchar(sel$text)}}")
  cli::cli_inform('
    range: [{sel$range$start["row"]}, {sel$range$start["column"]}] --
    [{sel$range$end["row"]}, {sel$range$end["column"]}]')
}

It's a little fiddly to get setup but here's what I do:

1. Define f() (at the end). (This is just for a pretty view of the current selection.)
2. Copy x <- rstudioapi::getSourceEditorContext() or just execute it once to get it in the history. (I'm using rstudioapi::getSourceEditorContext() instead of rstudioapi::getActiveDocumentContext() to make it easier to run the same code in RStudio, where those functions are actually different.)
3. Then follow the steps above, replacing the text in line 1 with the test cases above. I manually select the text inside the double quotes.