fix(lsp): make update_workspace_folders perform incremental updates per LSP spec (#7789)

Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>

Author: fronterior

.changeset/fair-crabs-play.md

@@ -0,0 +1,5 @@
+---
+"@biomejs/biome": patch
+---
+
+Fixed the LSP method `workspace/didChangeWorkspaceFolders` to perform incremental updates instead of replacing the entire folder list.

crates/biome_lsp/src/server.rs

@@ -408,7 +408,8 @@ impl LanguageServer for LSPServer {
             }
         }
 
-        self.session.update_workspace_folders(params.event.added);
+        self.session
+            .update_workspace_folders(params.event.added, params.event.removed);
         self.session.load_workspace_settings(true).await;
     }
 

crates/biome_lsp/src/session.rs

@@ -587,9 +587,24 @@ impl Session {
         self.workspace_folders.read().unwrap().clone()
     }
 
-    pub(crate) fn update_workspace_folders(&self, folders: Vec<WorkspaceFolder>) {
+    pub(crate) fn update_workspace_folders(
+        &self,
+        added: Vec<WorkspaceFolder>,
+        removed: Vec<WorkspaceFolder>,
+    ) {
         let mut workspace_folders = self.workspace_folders.write().unwrap();
-        *workspace_folders = Some(folders);
+
+        if let Some(ref mut folders) = *workspace_folders {
+            if !removed.is_empty() {
+                folders.retain(|folder| !removed.contains(folder));
+            }
+
+            if !added.is_empty() {
+                folders.extend(added);
+            }
+        } else {
+            *workspace_folders = Some(added);
+        }
     }
 
     /// Returns the base path of the workspace on the filesystem if it has one