fix: scrollbar gutters and dialog scrolling on open (#1967)

* fix: scrollbar gutters and dialog scrolling on open

* prettier

* fix check for current scrollbarGutter property

* prettier

docs/pages/resources/changelog.md

@@ -14,6 +14,7 @@ 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-tooltip>` that allowed unwanted text properties to leak in [#1947]
 
 ## 2.15.0

src/internal/scroll.ts

@@ -33,6 +33,19 @@ export function lockBodyScrolling(lockingEl: HTMLElement) {
   if (!document.documentElement.classList.contains('sl-scroll-lock')) {
     /** Scrollbar width + body padding calculation can go away once Safari has scrollbar-gutter support. */
     const scrollbarWidth = getScrollbarWidth() + getExistingBodyPadding(); // must be measured before the `sl-scroll-lock` class is applied
+
+    let scrollbarGutterProperty = getComputedStyle(document.documentElement).scrollbarGutter;
+
+    // default is auto, unsupported browsers is "undefined"
+    if (!scrollbarGutterProperty || scrollbarGutterProperty === 'auto') {
+      scrollbarGutterProperty = 'stable';
+    }
+
+    if (scrollbarWidth <= 0) {
+      // if there's no scrollbar, just set it to "revert" so whatever the user has set gets used. This is useful is the page is not overflowing and showing a scrollbar, or if the user has overflow: hidden, or any other reason a scrollbar may not be showing.
+      scrollbarGutterProperty = 'revert';
+    }
+    document.documentElement.style.setProperty('--sl-scroll-lock-gutter', scrollbarGutterProperty);
     document.documentElement.classList.add('sl-scroll-lock');
     document.documentElement.style.setProperty('--sl-scroll-lock-size', `${scrollbarWidth}px`);
   }

src/themes/_utility.css

@@ -6,7 +6,10 @@
 
 @supports (scrollbar-gutter: stable) {
   .sl-scroll-lock {
-    scrollbar-gutter: stable !important;
+    scrollbar-gutter: var(--sl-scroll-lock-gutter) !important;
+  }
+
+  .sl-scroll-lock body {
     overflow: hidden !important;
   }
 }