[Editor] Avoid to unselect some editors when the main window is focused

When several editors are selected and the window loses and then gets back its focus,
the previously focused editor is triggering its focus callback making it the only
selected one.
This patch aims to avoid triggering the focus callback called when the main window
gets its focus back.

src/display/editor/tools.js

@@ -544,12 +544,18 @@ class AnnotationEditorUIManager {
 
   #isEnabled = false;
 
+  #lastActiveElement = null;
+
   #mode = AnnotationEditorType.NONE;
 
   #selectedEditors = new Set();
 
   #pageColors = null;
 
+  #boundBlur = this.blur.bind(this);
+
+  #boundFocus = this.focus.bind(this);
+
   #boundCopy = this.copy.bind(this);
 
   #boundCut = this.cut.bind(this);
@@ -701,6 +707,7 @@ class AnnotationEditorUIManager {
 
   destroy() {
     this.#removeKeyboardManager();
+    this.#removeFocusManager();
     this.#eventBus._off("editingaction", this.#boundOnEditingAction);
     this.#eventBus._off("pagechanging", this.#boundOnPageChanging);
     this.#eventBus._off("scalechanging", this.#boundOnScaleChanging);
@@ -796,6 +803,50 @@ class AnnotationEditorUIManager {
     }
   }
 
+  #addFocusManager() {
+    window.addEventListener("focus", this.#boundFocus);
+    window.addEventListener("blur", this.#boundBlur);
+  }
+
+  #removeFocusManager() {
+    window.removeEventListener("focus", this.#boundFocus);
+    window.removeEventListener("blur", this.#boundBlur);
+  }
+
+  blur() {
+    if (!this.hasSelection) {
+      return;
+    }
+    // When several editors are selected and the window loses focus, we want to
+    // keep the last active element in order to be able to focus it again when
+    // the window gets the focus back but we don't want to trigger any focus
+    // callbacks else only one editor will be selected.
+    const { activeElement } = document;
+    for (const editor of this.#selectedEditors) {
+      if (editor.div.contains(activeElement)) {
+        this.#lastActiveElement = [editor, activeElement];
+        editor._focusEventsAllowed = false;
+        break;
+      }
+    }
+  }
+
+  focus() {
+    if (!this.#lastActiveElement) {
+      return;
+    }
+    const [lastEditor, lastActiveElement] = this.#lastActiveElement;
+    this.#lastActiveElement = null;
+    lastActiveElement.addEventListener(
+      "focusin",
+      () => {
+        lastEditor._focusEventsAllowed = true;
+      },
+      { once: true }
+    );
+    lastActiveElement.focus();
+  }
+
   #addKeyboardManager() {
     // The keyboard events are caught at the container level in order to be able
     // to execute some callbacks even if the current page doesn't have focus.
@@ -967,6 +1018,7 @@ class AnnotationEditorUIManager {
    */
   setEditingState(isEditing) {
     if (isEditing) {
+      this.#addFocusManager();
       this.#addKeyboardManager();
       this.#addCopyPasteListeners();
       this.#dispatchUpdateStates({
@@ -977,6 +1029,7 @@ class AnnotationEditorUIManager {
         hasSelectedEditor: false,
       });
     } else {
+      this.#removeFocusManager();
       this.#removeKeyboardManager();
       this.#removeCopyPasteListeners();
       this.#dispatchUpdateStates({