Fix: split panel properly recalculates when going from hidden to shown

docs/pages/resources/changelog.md

@@ -14,7 +14,8 @@ New versions of Shoelace are released as-needed and generally occur when a criti
 
 ## Next
 
-- Fixed a bug in `<dialog>` where when it showed it would create a layout shift. [#1967]
+- Fixed a bug in `<sl-split-panel>` that caused it not to recalculate it's position when going from being `display: none;` to its original display value. [#1942]
+- Fixed a bug in `<dialog>` where when it showed it would cause a layout shift. [#1967]
 - Fixed a bug in `<sl-tooltip>` that allowed unwanted text properties to leak in [#1947]
 
 ## 2.15.0

src/components/split-panel/split-panel.component.ts

@@ -189,6 +189,14 @@ export default class SlSplitPanel extends ShoelaceElement {
     const { width, height } = entries[0].contentRect;
     this.size = this.vertical ? height : width;
 
+    // There's some weird logic that gets `this.cachedPositionInPixels = NaN` or `this.position === Infinity` when
+    // a split-panel goes from `display: none;` to showing.
+    if (isNaN(this.cachedPositionInPixels) || this.position === Infinity) {
+      this.cachedPositionInPixels = Number(this.getAttribute('position-in-pixels'));
+      this.positionInPixels = Number(this.getAttribute('position-in-pixels'));
+      this.position = this.pixelsToPercentage(this.positionInPixels);
+    }
+
     // Resize when a primary panel is set
     if (this.primary) {
       this.position = this.pixelsToPercentage(this.cachedPositionInPixels);