Enables every combination of TextEdit and LayoutJob alignments

[RndUsr123]

This is a fix/improvement that makes all kinds of alignments work in TextEdit when a custom LayoutJob with halign is used.

I used the simplest approach possible to avoid unwanted bugs as I wasn't sure what's safe to change, but there's potentially better ways to achieve this. In particular, I'm not sure I fully understand the rationale behind aligning rows in a Galley based on the latter's leftmost border, considering the size is what's ultimately used in widgets (in TextEdit at least).

Regardless, here's a demo of this PR:

text_edit.mp4

• I have followed the instructions in the PR template

[github-actions]

Preview available at https://egui-pr-preview.github.io/pr/7831-TextEdit-fix
Note that it might take a couple seconds for the update to show up after the preview_build workflow has completed.

View snapshot changes at kitdiff

[emilk]

Very nice; thank you! Could you please also add a snapshot test for this? 🙏

See tests/egui_tests/tests/regression_tests.rs for a template to follow (put yours in tests/egui_tests/tests/test_text_edit.rs)

[RndUsr123]

That could be tricky because there's no way to determine where the text has been rendered after the fact from what I can gather. The main issue stems from the fact TextEdit relies on the Galley's rect size rather than its alignment; I'm a bit wary of changing how Galley is computed as that could introduce other bugs so I just visually shifted the text instead.
Here's what I mean for reference (the boxes show the Galley rects):

I've also just found another bug in LayoutJob which prevents the cursor from moving forward after a trailing space when halign is RIGHT or Center, so that would probably need to be fixed before a proper test about cursor position can be written. Should I open another issue for it or should it be part of this PR?

Edit: this is what it looks like (it was a thing before my fix)

demo.mp4

[emilk]

You don't need to test text editing (unless you want to). Just adding your nice 3x3 matrix and snapshot-testing that would be great. See for instance this:

egui/tests/egui_tests/tests/regression_tests.rs

Lines 38 to 63 in f26d7d8

	#[test]
	fn text_edit_rtl() {
	let mut text = "hello ".to_owned();
	let mut harness = Harness::builder().with_size((200.0, 50.0)).build_ui(|ui| {
	ui.with_layout(Layout::right_to_left(Align::Min), |ui| {
	_ = ui.button("right");
	ui.add(
	egui::TextEdit::singleline(&mut text)
	.desired_width(10.0)
	.clip_text(false),
	);
	_ = ui.button("left");
	});
	});
	harness.get_by_role(Role::TextInput).focus();
	harness.step();
	harness.snapshot("text_edit_rtl_0");
	harness.get_by_role(Role::TextInput).type_text("world");
	for i in 1..3 {
	harness.step();
	harness.snapshot(format!("text_edit_rtl_{i}"));
	}
	}

[RndUsr123]

Oh I see... sure I'll do it!

Btw I think I've found out how to fix the LayoutJob issue: it was trimming them out even when the row didn't need to be justified so changing that apparently fixed the problem and justify still works:

This is what I'm going for:

let [original_min_x, original_max_x];

if justify {
    original_min_x = row.glyphs[glyph_range[0]].logical_rect().min.x;
    original_max_x = row.glyphs[glyph_range[1] - 1].logical_rect().max.x;
}
else {
    original_min_x = row.glyphs[0].logical_rect().min.x;
    original_max_x = row.glyphs[row.glyphs.len() - 1].logical_rect().max.x;
}

let original_width = original_max_x - original_min_x;

would you prefer a direct assignment or something else style-wise?

[RndUsr123]

I've added the regression test! Ended up settling for a single snapshot, which should be good enough to showcase both visual alignment in a few scenarios as well as correct cursor placement.
The LayoutJob whitespace trim situation turned out to be not as trivial so I'll open a new issue for that.