refac

src/lib/components/common/DiffEditorModal.svelte

@@ -14,7 +14,12 @@
 	import ConfirmDialog from './ConfirmDialog.svelte';
 	import FileCodeEditor from '$lib/components/chat/FileNav/FileCodeEditor.svelte';
 	import XMark from '$lib/components/icons/XMark.svelte';
-	import { lineAlignment, alignChangedLines, clearLineAlignment } from './diffEditorAlignment';
+	import {
+		lineAlignment,
+		alignChangedLines,
+		clearLineAlignment,
+		pairedLinePosition
+	} from './diffEditorAlignment';
 	import {
 		copyChunk,
 		selectedChunk,
@@ -222,10 +227,32 @@
 								(alignmentDocuments[0] !== documents[0] ||
 									alignmentDocuments[1] !== documents[1] ||
 									geometry !== alignmentGeometry);
+							const resized = alignmentDocuments && geometry !== alignmentGeometry;
 							alignmentDocuments = documents;
 							alignmentGeometry = geometry;
 							if (!reset || !clearLineAlignment(view.a, view.b))
 								alignChangedLines(view.a, view.b, view.chunks);
+							if (resized) {
+								// Reflow can leave one viewport entirely inside an old EOF spacer.
+								// Measure both panes around the row the reader was looking at.
+								const top = Math.max(0, node.getBoundingClientRect().top - view.a.documentTop);
+								const position = Math.min(
+									view.a.lineBlockAtHeight(top).from,
+									view.a.state.doc.length
+								);
+								const other = pairedLinePosition(
+									view.a.state.doc,
+									view.b.state.doc,
+									view.chunks,
+									position
+								);
+								view.a.dispatch({
+									effects: EditorView.scrollIntoView(position, { y: 'start', yMargin: 0 })
+								});
+								view.b.dispatch({
+									effects: EditorView.scrollIntoView(other, { y: 'start', yMargin: 0 })
+								});
+							}
 							view.reconfigure({});
 						}
 					});

src/lib/components/common/diffEditorAlignment.ts

@@ -1,4 +1,4 @@
-import { StateEffect, StateField } from '@codemirror/state';
+import { StateEffect, StateField, type Text } from '@codemirror/state';
 import {
 	BlockType,
 	Decoration,
@@ -8,6 +8,21 @@ import {
 } from '@codemirror/view';
 import type { Chunk } from '@codemirror/merge';
 
+export function pairedLinePosition(a: Text, b: Text, chunks: readonly Chunk[], position: number) {
+	let offset = 0;
+	for (const chunk of chunks) {
+		if (position < chunk.fromA) break;
+		if (position < chunk.toA) {
+			const row = a.lineAt(position).number - a.lineAt(chunk.fromA).number;
+			const start = b.lineAt(Math.min(chunk.fromB, b.length)).number;
+			const end = b.lineAt(Math.min(Math.max(chunk.fromB, chunk.toB - 1), b.length)).number;
+			return b.line(Math.min(start + row, end)).from;
+		}
+		offset = chunk.toB - chunk.toA;
+	}
+	return Math.max(0, Math.min(position + offset, b.length));
+}
+
 class RowSpacer extends WidgetType {
 	constructor(readonly height: number) {
 		super();
@@ -65,7 +80,9 @@ export function alignChangedLines(a: EditorView, b: EditorView, chunks: readonly
 		const block = view.lineBlockAt(pos);
 		const text = Array.isArray(block.type)
 			? block.type.find((part) => part.type === BlockType.Text && part.from === pos)
-			: block.type === BlockType.Text ? block : null;
+			: block.type === BlockType.Text
+				? block
+				: null;
 		return text?.from === pos ? text.top : null;
 	};
 	const align = (posA: number, posB: number) => {