fix: invalidate overlay content size cache on positionTarget change

The PositionMixin caches the largest content size ever measured via
`Math.max(__oldContentWidth, offsetWidth)` to keep alignment stable
under window-resize squeeze. The cache was never invalidated when
`positionTarget` changed, so an overlay that switched to a target with
smaller content (e.g. a tooltip moving from a long-text item to a
short-text one) kept calculating fit against the previous larger size
and stayed flipped to the wrong side.

Reset `__oldContentWidth` and `__oldContentHeight` whenever
`positionTarget` changes so the next `_updatePosition` measures the
overlay against its current content.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

Author: vursen

packages/overlay/src/vaadin-overlay-position-mixin.js

@@ -145,6 +145,12 @@ export const PositionMixin = (superClass) =>
       if (props.has('positionTarget')) {
         const oldTarget = props.get('positionTarget');
 
+        // Invalidate the cached content size so the next `_updatePosition` call
+        // measures the overlay against the new target instead of carrying over
+        // a larger size from the previous one.
+        this.__oldContentWidth = undefined;
+        this.__oldContentHeight = undefined;
+
         // 1. When position target is removed, always reset position settings
         // 2. When position target is set, reset if overlay was opened before
         if ((!this.positionTarget && oldTarget) || (this.positionTarget && !oldTarget && !!this.__margins)) {

packages/overlay/test/position-mixin.test.js

@@ -511,6 +511,30 @@ describe('position mixin', () => {
       expect(parent.hasAttribute('end-aligned')).to.be.true;
     });
 
+    it('should re-evaluate alignment when positionTarget changes after content shrinks', async () => {
+      // Make the content wide enough to force a flip at a target position
+      // where the narrow content would still fit.
+      overlayContent.style.width = '200px';
+      const wideFlipThreshold = document.documentElement.clientWidth - 200 - margin;
+      target.style.left = `${wideFlipThreshold + 3}px`;
+      updatePosition();
+      expect(overlay.hasAttribute('end-aligned')).to.be.true;
+
+      // Shrink the content so the same target position fits without a flip.
+      overlayContent.style.width = '50px';
+
+      // Switch to a fresh target at the same position; assert right after
+      // the synchronous Lit update so we observe the cached-size behavior
+      // before the deferred `requestAnimationFrame` re-position kicks in.
+      const newTarget = target.cloneNode(true);
+      target.parentElement.appendChild(newTarget);
+      overlay.positionTarget = newTarget;
+      await overlay.updateComplete;
+
+      expect(overlay.hasAttribute('start-aligned')).to.be.true;
+      expect(overlay.hasAttribute('end-aligned')).to.be.false;
+    });
+
     describe('no overlap', () => {
       beforeEach(() => {
         overlay.noHorizontalOverlap = true;