diff --git a/src/helpers/helpers.dom.ts b/src/helpers/helpers.dom.ts
index b0c41eec0c0..ddba06e6b50 100644
--- a/src/helpers/helpers.dom.ts
+++ b/src/helpers/helpers.dom.ts
@@ -191,9 +191,19 @@ export function getMaximumSize(
 
   const maintainHeight = bbWidth !== undefined || bbHeight !== undefined;
 
-  if (maintainHeight && aspectRatio && containerSize.height && height > containerSize.height) {
-    height = containerSize.height;
-    width = round1(Math.floor(height * aspectRatio));
+  if (maintainHeight && aspectRatio) {
+    // An auto-height container is just the canvas's own height read back, so it must not clamp the chart
+    // See https://github.com/chartjs/Chart.js/issues/12251
+    const liveHeight = getContainerSize(canvas, undefined, undefined).height;
+    const parent = canvas.parentElement;
+    const containerFollowsCanvas = liveHeight !== undefined
+      && Math.abs(liveHeight - canvas.clientHeight) < 1
+      && !(parent && parent.style.height);
+
+    if (!containerFollowsCanvas && liveHeight && height > liveHeight) {
+      height = liveHeight;
+      width = round1(height * aspectRatio);
+    }
   }
 
   return {width, height};
diff --git a/test/specs/issue.12251.tests.js b/test/specs/issue.12251.tests.js
new file mode 100644
index 00000000000..17fedf0724a
--- /dev/null
+++ b/test/specs/issue.12251.tests.js
@@ -0,0 +1,176 @@
+// https://github.com/chartjs/Chart.js/issues/12251
+// An auto-height container with maintainAspectRatio could only ever shrink.
+describe('Issue #12251 - chart follows an auto-height container in both directions', function() {
+  let originalDescriptor;
+
+  beforeEach(function() {
+    originalDescriptor = Object.getOwnPropertyDescriptor(window, 'devicePixelRatio');
+  });
+
+  afterEach(function() {
+    if (originalDescriptor) {
+      Object.defineProperty(window, 'devicePixelRatio', originalDescriptor);
+    } else {
+      delete window.devicePixelRatio;
+    }
+  });
+
+  function acquire(onResize) {
+    return acquireChart({
+      type: 'line',
+      data: {labels: ['a', 'b', 'c'], datasets: [{data: [1, 2, 3]}]},
+      options: {
+        responsive: true,
+        maintainAspectRatio: true,
+        animation: false,
+        onResize
+      }
+    }, {
+      canvas: {style: ''},
+      // No explicit height: the container is as tall as the canvas, as on chartjs.org.
+      wrapper: {style: 'width: 300px; position: relative'}
+    });
+  }
+
+  [1, 1.35].forEach(function(dpr) {
+    it('follows the container both when it grows and when it shrinks at DPR ' + dpr, function(done) {
+      Object.defineProperty(window, 'devicePixelRatio', {configurable: true, get: () => dpr});
+      const resizes = [];
+      const chart = acquire((c, size) => resizes.push({...size, cw: c.canvas.width, ch: c.canvas.height}));
+      const wrapper = chart.canvas.parentNode;
+      const tracked = [];
+
+      // Widths chosen so width * DPR is not an integer, which is where rounding bites.
+      const widths = [333.3, 401.7, 250.1, 377.9];
+      let step = 0;
+
+      function next() {
+        if (step === widths.length) {
+          // The bug: growth was refused while shrink was honoured, so the chart
+          // could only ever get smaller. Every change must be followed, both ways.
+          tracked.forEach(({target, got}) => {
+            expect(Math.abs(got - target)).toBeLessThan(1, `chart.width ${got} should follow container ${target}`);
+          });
+          done();
+          return;
+        }
+        wrapper.style.width = widths[step++] + 'px';
+        // Give the observer several frames to settle so any ping-pong is counted.
+        setTimeout(() => {
+          const target = parseFloat(wrapper.style.width);
+          tracked.push({target, got: chart.width});
+          next();
+        }, 250);
+      }
+      next();
+    });
+  });
+
+  it('does not lose size across repeated identical resizes', function(done) {
+    Object.defineProperty(window, 'devicePixelRatio', {configurable: true, get: () => 1.35});
+    const chart = acquire();
+    const wrapper = chart.canvas.parentNode;
+    let i = 0;
+    const seen = [];
+
+    function bounce() {
+      if (i === 12) {
+        const distinct = new Set(seen);
+        expect(distinct.size).toBe(1, 'same container width must always yield the same canvas size, got: ' + [...distinct].join(','));
+        done();
+        return;
+      }
+      // Alternate between two widths; every return to 333.3 must give the same result.
+      wrapper.style.width = (i % 2 === 0 ? 333.3 : 400) + 'px';
+      setTimeout(() => {
+        if (i % 2 === 0) {
+          seen.push(chart.canvas.width);
+        }
+        i++;
+        bounce();
+      }, 150);
+    }
+    bounce();
+  });
+
+  it('stays stable while a looping animation keeps the animator running', function(done) {
+    // A looping animation keeps the animator running, so every resize is deferred.
+    Object.defineProperty(window, 'devicePixelRatio', {configurable: true, get: () => 1.35});
+    const chart = acquireChart({
+      type: 'line',
+      data: {labels: ['a', 'b', 'c'], datasets: [{data: [1, 2, 3]}]},
+      options: {
+        responsive: true,
+        maintainAspectRatio: true,
+        animations: {tension: {duration: 200, easing: 'linear', from: 1, to: 0, loop: true}}
+      }
+    }, {
+      canvas: {style: ''},
+      wrapper: {style: 'width: 300px; position: relative'}
+    });
+    const wrapper = chart.canvas.parentNode;
+    const widths = [];
+
+    // Hold the container still and watch: a stable chart keeps one width.
+    setTimeout(() => {
+      wrapper.style.width = '380px';
+      const sample = setInterval(() => widths.push(chart.width), 100);
+      setTimeout(() => {
+        clearInterval(sample);
+        const settled = widths.slice(-5);
+        expect(new Set(settled).size).toBe(1, 'chart must settle to one width, got: ' + settled.join(','));
+        expect(Math.abs(settled[0] - 380)).toBeLessThan(1, 'chart must follow the container to 380px');
+        chart.destroy();
+        done();
+      }, 1500);
+    }, 400);
+  });
+
+  // With the animator running, resizes are deferred and applied with a stale height,
+  // which used to be mistaken for a fixed container and clamped to.
+  it('grows back after shrinking, instead of clamping to its own previous height', function(done) {
+    const chart = acquireChart({
+      type: 'line',
+      data: {labels: ['a', 'b', 'c'], datasets: [{data: [1, 2, 3]}]},
+      options: {
+        responsive: true,
+        maintainAspectRatio: true,
+        animations: {tension: {duration: 200, easing: 'linear', from: 1, to: 0, loop: true}}
+      }
+    }, {canvas: {style: ''}, wrapper: {style: 'width: 552.6px; position: relative'}});
+    const wrapper = chart.canvas.parentNode;
+    const trace = [];
+
+    function drag(path, then) {
+      let i = 0;
+      (function frame() {
+        if (i < path.length) {
+          wrapper.style.width = path[i++] + 'px';
+          requestAnimationFrame(frame);
+        } else {
+          setTimeout(then, 400);
+        }
+      }());
+    }
+    const steps = (from, to, by) => {
+      const out = [];
+      for (let w = from; by > 0 ? w <= to : w >= to; w += by) {
+        out.push(+w.toFixed(1));
+      }
+      return out;
+    };
+
+    setTimeout(() => {
+      drag(steps(552.6, 360.6, -6), () => {
+        trace.push(`after shrink: container ${wrapper.getBoundingClientRect().width.toFixed(1)} chart ${chart.width}`);
+        drag(steps(360.6, 614, 6), () => {
+          const cw = wrapper.getBoundingClientRect().width;
+          trace.push(`after grow:   container ${cw.toFixed(1)} chart ${chart.width}`);
+          expect(Math.abs(chart.width - cw)).toBeLessThan(1, `chart ${chart.width} must follow the container back up to ${cw.toFixed(1)}`);
+          chart.destroy();
+          done();
+        });
+      });
+    }, 300);
+  });
+});