Discard rendered pages while the reader is in a hidden tab

src/common/reader.js

@@ -2167,6 +2167,14 @@ class Reader {
 		this._updateState({ freeze: false });
 	}
 
+	// Release rendered pages and pause rendering while the reader is in a
+	// hidden (background) tab, and restore them when the tab is shown again.
+	// Currently only has an effect for the PDF view
+	setSuspended(suspended) {
+		this._primaryView?.setSuspended?.(suspended);
+		this._secondaryView?.setSuspended?.(suspended);
+	}
+
 	print() {
 		if (this._type === 'pdf') {
 			if (this._state.annotations.length) {

src/pdf/page.js

@@ -57,6 +57,11 @@ export default class Page {
 		this._pageRenderer.render();
 	}
 
+	destroy() {
+		this._pageRenderer.destroy();
+		this._detailRenderer.destroy();
+	}
+
 	renderAnnotationOnCanvas(annotation, canvas) {
 		return this._pageRenderer.renderAnnotationOnCanvas(annotation, canvas);
 	}
@@ -95,6 +100,16 @@ class Renderer {
 			: this._originalPage.canvas;
 	}
 
+	// Zero the snapshot canvas to release its backing store immediately,
+	// rather than waiting for GC
+	destroy() {
+		this._snapshotCanvas.width = 0;
+		this._snapshotCanvas.height = 0;
+		this._lastSourceCanvas = null;
+		this._lastSourceSize = { w: 0, h: 0 };
+		this._context = null;
+	}
+
 	_initContext() {
 		let baseCanvas = this._getSourceCanvas();
 		this._context = baseCanvas ? baseCanvas.getContext('2d') : null;

src/pdf/pdf-view.js

@@ -682,6 +682,11 @@ class PDFView {
 
 	_handlePageDestroy(originalPage) {
 		let pageIndex = originalPage.id - 1;
+		for (let page of this._pages) {
+			if (page.originalPage === originalPage) {
+				page.destroy();
+			}
+		}
 		this._pages = this._pages.filter(x => x.originalPage !== originalPage);
 		delete this._pdfPages[pageIndex];
 	}
@@ -841,6 +846,36 @@ class PDFView {
 		this._overlayPopupDelayer.destroy();
 	}
 
+	// Discard rendered pages while the view is hidden (i.e. in a background tab)
+	// to release page canvases, page snapshot canvases and worker-side page
+	// resources, which otherwise are retained for up to 10 pages per view.
+	// Page views stay in place and visible pages re-render on resume
+	setSuspended(suspended) {
+		suspended = !!suspended;
+		if (this._suspended === suspended) {
+			return;
+		}
+		this._suspended = suspended;
+		let app = this._iframeWindow?.PDFViewerApplication;
+		if (!app?.pdfViewer) {
+			return;
+		}
+		if (suspended) {
+			// Prevent new rendering from starting while hidden, otherwise a resize
+			// or scale change would immediately re-render the discarded pages
+			app.pdfRenderingQueue.paused = true;
+			for (let pageView of app.pdfViewer._pages) {
+				pageView.destroy();
+			}
+			// Release worker-side resources (fonts, images) until re-rendering
+			app.pdfDocument?.cleanup().catch(() => {});
+		}
+		else {
+			app.pdfRenderingQueue.paused = false;
+			app.pdfViewer.update();
+		}
+	}
+
 	focus() {
 		this._iframe.focus();
 		// this._iframeWindow.focus();