diff --git a/packages/joint-core/src/dia/Paper.mjs b/packages/joint-core/src/dia/Paper.mjs
index affdcd98ad..f70ce26ccd 100644
--- a/packages/joint-core/src/dia/Paper.mjs
+++ b/packages/joint-core/src/dia/Paper.mjs
@@ -668,6 +668,7 @@ export const Paper = View.extend({
         this.cloneOptions();
         this.render();
         this._setDimensions();
+        this._startObservingElementSize();
         this.startListening();
 
         // Mouse wheel events buffer
@@ -2253,6 +2254,7 @@ export const Paper = View.extend({
 
     onRemove: function() {
 
+        this._stopObservingElementSize();
         this.freeze();
         this._updates.disabled = true;
         //clean up all DOM elements/views to prevent memory leaks
@@ -2281,6 +2283,10 @@ export const Paper = View.extend({
         this._setDimensions();
         const computedSize = this.getComputedSize();
         this.trigger('resize', computedSize.width, computedSize.height, data);
+        // Re-evaluate observer attachment in case the user toggled between
+        // explicit-size and CSS-relative modes.
+        this._stopObservingElementSize();
+        this._startObservingElementSize();
     },
 
     _setDimensions: function() {
@@ -2295,6 +2301,47 @@ export const Paper = View.extend({
         });
     },
 
+    _elementResizeObserver: null,
+    _lastObservedSize: null,
+
+    _startObservingElementSize: function() {
+        if (this._elementResizeObserver) return;
+        if (typeof ResizeObserver === 'undefined') return;
+        const { width, height } = this.options;
+        // Explicit-size mode: `setDimensions()` is the sole source of 'resize'.
+        if (isNumber(width) && isNumber(height)) return;
+
+        const size = this.getComputedSize();
+        this._lastObservedSize = { width: size.width, height: size.height };
+
+        this._elementResizeObserver = new ResizeObserver(() => {
+            if (!this.el || !this._elementResizeObserver) return;
+            // Re-read via getComputedSize() rather than the ResizeObserverEntry
+            // payload. `clientWidth/clientHeight` (the non-numeric branch of
+            // getComputedSize) is the padding box minus scrollbar — none of
+            // `contentBoxSize` / `borderBoxSize` / `contentRect` match exactly
+            // once the host has padding or border. Re-reading keeps the size
+            // we emit identical to every other `getComputedSize()` caller; the
+            // cost is one DOM read in a post-layout callback.
+            const next = this.getComputedSize();
+            const prev = this._lastObservedSize;
+            if (prev && prev.width === next.width && prev.height === next.height) return;
+            this._lastObservedSize = { width: next.width, height: next.height };
+            // The host already has its CSS-driven size; do not call
+            // `_setDimensions()` here — writing px values back would clobber CSS.
+            this.trigger('resize', next.width, next.height, { source: 'observer' });
+        });
+        this._elementResizeObserver.observe(this.el);
+    },
+
+    _stopObservingElementSize: function() {
+        if (this._elementResizeObserver) {
+            this._elementResizeObserver.disconnect();
+            this._elementResizeObserver = null;
+        }
+        this._lastObservedSize = null;
+    },
+
     // Expand/shrink the paper to fit the content.
     // Alternatively signature function(opt)
     fitToContent: function(gridWidth, gridHeight, padding, opt) {
diff --git a/packages/joint-core/test/jointjs/paper.js b/packages/joint-core/test/jointjs/paper.js
index d35166fe7e..e5fd1db86e 100644
--- a/packages/joint-core/test/jointjs/paper.js
+++ b/packages/joint-core/test/jointjs/paper.js
@@ -95,6 +95,79 @@ QUnit.module('paper', function(hooks) {
             resizeCbSpy.resetHistory();
         });
 
+        QUnit.module('autoResizePaper', function() {
+
+            // Waits for the ResizeObserver to deliver its async callback.
+            // Two rAFs cover the spec's "delivered before the next paint" plus
+            // browser implementation slack.
+            function afterResizeObserverDelivery(callback) {
+                requestAnimationFrame(function() {
+                    requestAnimationFrame(callback);
+                });
+            }
+
+            QUnit.test('fires resize when host CSS size changes', function(assert) {
+                var done = assert.async();
+                var paper = this.paper;
+                // '100%' / '100%' makes the paper fill the container while
+                // keeping options non-numeric (i.e. observer-eligible).
+                $container.css({ width: '200px', height: '100px' });
+                paper.setDimensions('100%', '100%');
+                var spy = sinon.spy();
+                paper.on('resize', spy);
+                afterResizeObserverDelivery(function() {
+                    spy.resetHistory();
+                    $container.css({ width: '300px', height: '150px' });
+                    afterResizeObserverDelivery(function() {
+                        assert.ok(spy.called, 'resize fires for host size change');
+                        var args = spy.lastCall.args;
+                        assert.equal(args[0], 300, 'width reports host clientWidth');
+                        assert.equal(args[1], 150, 'height reports host clientHeight');
+                        assert.deepEqual(args[2], { source: 'observer' }, 'data carries observer source');
+                        done();
+                    });
+                });
+            });
+
+            QUnit.test('numeric dimensions skip the observer', function(assert) {
+                var done = assert.async();
+                var paper = this.paper;
+                paper.setDimensions(200, 100);
+                var spy = sinon.spy();
+                paper.on('resize', spy);
+                $container.css({ width: '500px', height: '500px' });
+                afterResizeObserverDelivery(function() {
+                    assert.ok(spy.notCalled, 'host CSS resize ignored in explicit-size mode');
+                    done();
+                });
+            });
+
+            QUnit.test('disconnects on paper.remove()', function(assert) {
+                var paperEl = document.createElement('div');
+                $container.append(paperEl);
+                var paper = new joint.dia.Paper({
+                    el: paperEl,
+                    model: new joint.dia.Graph,
+                    width: '100%',
+                    height: '100%'
+                });
+                assert.ok(paper._elementResizeObserver, 'observer attached on init');
+                var disconnectSpy = sinon.spy(paper._elementResizeObserver, 'disconnect');
+                paper.remove();
+                assert.ok(disconnectSpy.calledOnce, 'disconnect called exactly once on remove');
+                assert.notOk(paper._elementResizeObserver, 'observer reference cleared');
+            });
+
+            QUnit.test('toggling between fixed and CSS-relative dimensions attaches/detaches observer', function(assert) {
+                var paper = this.paper;
+                assert.notOk(paper._elementResizeObserver, 'no observer with default numeric dims');
+                paper.setDimensions('100%', '100%');
+                assert.ok(paper._elementResizeObserver, 'observer attached after switching to CSS-relative');
+                paper.setDimensions(100, 100);
+                assert.notOk(paper._elementResizeObserver, 'observer detached when both dims numeric again');
+            });
+        });
+
     });
 
     QUnit.test('paper.addCell() number of sort()', function(assert) {