diff --git a/src/vs/base/common/map.ts b/src/vs/base/common/map.ts
index 132b8f79e65..b45efcf3784 100644
--- a/src/vs/base/common/map.ts
+++ b/src/vs/base/common/map.ts
@@ -741,13 +741,17 @@ interface ResourceMapKeyFn {
 	(resource: URI): string;
 }
 
+class ResourceMapEntry<T> {
+	constructor(readonly uri: URI, readonly value: T) { }
+}
+
 export class ResourceMap<T> implements Map<URI, T> {
 
 	private static readonly defaultToKey = (resource: URI) => resource.toString();
 
 	readonly [Symbol.toStringTag] = 'ResourceMap';
 
-	private readonly map: Map<string, T>;
+	private readonly map: Map<string, ResourceMapEntry<T>>;
 	private readonly toKey: ResourceMapKeyFn;
 
 	/**
@@ -774,12 +778,12 @@ export class ResourceMap<T> implements Map<URI, T> {
 	}
 
 	set(resource: URI, value: T): this {
-		this.map.set(this.toKey(resource), value);
+		this.map.set(this.toKey(resource), new ResourceMapEntry(resource, value));
 		return this;
 	}
 
 	get(resource: URI): T | undefined {
-		return this.map.get(this.toKey(resource));
+		return this.map.get(this.toKey(resource))?.value;
 	}
 
 	has(resource: URI): boolean {
@@ -802,30 +806,32 @@ export class ResourceMap<T> implements Map<URI, T> {
 		if (typeof thisArg !== 'undefined') {
 			clb = clb.bind(thisArg);
 		}
-		for (let [index, value] of this.map) {
-			clb(value, URI.parse(index), <any>this);
+		for (let [_, entry] of this.map) {
+			clb(entry.value, entry.uri, <any>this);
 		}
 	}
 
-	values(): IterableIterator<T> {
-		return this.map.values();
+	*values(): IterableIterator<T> {
+		for (let entry of this.map.values()) {
+			yield entry.value;
+		}
 	}
 
 	*keys(): IterableIterator<URI> {
-		for (let key of this.map.keys()) {
-			yield URI.parse(key);
+		for (let entry of this.map.values()) {
+			yield entry.uri;
 		}
 	}
 
 	*entries(): IterableIterator<[URI, T]> {
-		for (let tuple of this.map.entries()) {
-			yield [URI.parse(tuple[0]), tuple[1]];
+		for (let entry of this.map.values()) {
+			yield [entry.uri, entry.value];
 		}
 	}
 
 	*[Symbol.iterator](): IterableIterator<[URI, T]> {
-		for (let item of this.map) {
-			yield [URI.parse(item[0]), item[1]];
+		for (let [, entry] of this.map) {
+			yield [entry.uri, entry.value];
 		}
 	}
 }
diff --git a/src/vs/base/test/common/map.test.ts b/src/vs/base/test/common/map.test.ts
index fc0f1fc0034..633749fba96 100644
--- a/src/vs/base/test/common/map.test.ts
+++ b/src/vs/base/test/common/map.test.ts
@@ -1189,6 +1189,25 @@ suite('Map', () => {
 		assert.strictEqual(map.get(windowsFile), 'true');
 		assert.strictEqual(map.get(uncFile), 'true');
 	});
+
+	test('ResourceMap - files (ignorecase, BUT preservecase)', function () {
+		const map = new ResourceMap<number>(uri => extUriIgnorePathCase.getComparisonKey(uri));
+
+		const fileA = URI.parse('file://some/filea');
+		const fileAUpper = URI.parse('file://SOME/FILEA');
+
+		map.set(fileA, 1);
+		assert.strictEqual(map.get(fileA), 1);
+		assert.strictEqual(map.get(fileAUpper), 1);
+		assert.deepStrictEqual(Array.from(map.keys()).map(String), [fileA].map(String));
+		assert.deepStrictEqual(Array.from(map), [[fileA, 1]]);
+
+		map.set(fileAUpper, 1);
+		assert.strictEqual(map.get(fileA), 1);
+		assert.strictEqual(map.get(fileAUpper), 1);
+		assert.deepStrictEqual(Array.from(map.keys()).map(String), [fileAUpper].map(String));
+		assert.deepStrictEqual(Array.from(map), [[fileAUpper, 1]]);
+	});
 });
 
 
diff --git a/src/vs/workbench/api/common/extHostDiagnostics.ts b/src/vs/workbench/api/common/extHostDiagnostics.ts
index 00038bd692d..bacfe4bdd80 100644
--- a/src/vs/workbench/api/common/extHostDiagnostics.ts
+++ b/src/vs/workbench/api/common/extHostDiagnostics.ts
@@ -16,13 +16,12 @@ import { ResourceMap } from 'vs/base/common/map';
 import { ExtensionIdentifier } from 'vs/platform/extensions/common/extensions';
 import { IExtHostFileSystemInfo } from 'vs/workbench/api/common/extHostFileSystemInfo';
 import { IExtUri } from 'vs/base/common/resources';
-import { SkipList } from 'vs/base/common/skipList';
 
 export class DiagnosticCollection implements vscode.DiagnosticCollection {
 
 	readonly #proxy: MainThreadDiagnosticsShape | undefined;
 	readonly #onDidChangeDiagnostics: Emitter<vscode.Uri[]>;
-	readonly #data: SkipList<URI, vscode.Diagnostic[]>;
+	readonly #data: ResourceMap<vscode.Diagnostic[]>;
 
 	private _isDisposed = false;
 
@@ -34,7 +33,7 @@ export class DiagnosticCollection implements vscode.DiagnosticCollection {
 		proxy: MainThreadDiagnosticsShape | undefined,
 		onDidChangeDiagnostics: Emitter<vscode.Uri[]>
 	) {
-		this.#data = new SkipList((a, b) => extUri.compare(a, b));
+		this.#data = new ResourceMap(uri => extUri.getComparisonKey(uri));
 		this.#proxy = proxy;
 		this.#onDidChangeDiagnostics = onDidChangeDiagnostics;
 	}