[Repo Assist] Use scrollbar-gutter: stable on scroll containers (#1088)

* Use scrollbar-gutter: stable on scroll containers

Add scrollbar-gutter: stable (Baseline 2024) to all vertical-scroll
containers to reserve scrollbar gutter space and prevent layout shifts:

- main: also adds missing overflow-y: auto so long pages are
  independently scrollable (html has overflow-y: hidden, so main
  must be its own scroll container)
- #fsdocs-main-menu: sidebar no longer reflows content when
  the scrollbar appears/disappears
- Mobile .menu dropdown: same benefit
- dialog ul (search results): prevents width shift when results
  exceed max-height

Closes #1087

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* ci: trigger checks

* Bump RELEASE_NOTES: promote Unreleased to 22.0.0-alpha.2

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* ci: trigger checks

---------

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Florian Verdonck <florian.verdonck@outlook.com>

Author: 3 people

RELEASE_NOTES.md

@@ -2,6 +2,8 @@
 
 ## [Unreleased]
 
+## 22.0.0-alpha.2 - 2026-03-13
+
 ### Added
 * Search dialog now auto-focuses the search input when opened, clears on close, and can be triggered with `Ctrl+K` / `Cmd+K` in addition to `/`.
 * Add `dotnet fsdocs convert` command to convert a single `.md`, `.fsx`, or `.ipynb` file to HTML (or another output format) without building a full documentation site. [#811](https://github.com/fsprojects/FSharp.Formatting/issues/811)
@@ -14,6 +16,7 @@
 * Generated code tokens no longer use inline `onmouseover`/`onmouseout` event handlers. Tooltips are now triggered via `data-fsdocs-tip` / `data-fsdocs-tip-unique` attributes and a delegated event listener in `fsdocs-tips.js`. The `popover` attribute is also added to API-doc tooltip divs so they use the same top-layer path. [#1061](https://github.com/fsprojects/FSharp.Formatting/pull/1061)
 * Changed `range` fields in `MarkdownSpan` and `MarkdownParagraph` DU cases from `MarkdownRange option` to `MarkdownRange`, using `MarkdownRange.zero` as the default/placeholder value instead of `None`.
 * When no template is provided (e.g. `fsdocs convert` without `--template`), `fsdocs-tip` tooltip divs are no longer included in the output. Tooltips require JavaScript/CSS from a template to function, so omitting them produces cleaner raw output. [#1019](https://github.com/fsprojects/FSharp.Formatting/pull/1019)
+* Use [`scrollbar-gutter: stable`](https://developer.mozilla.org/en-US/docs/Web/CSS/scrollbar-gutter) (Baseline 2024) on scroll containers (`main`, `#fsdocs-main-menu`, mobile menu, search dialog) to reserve scrollbar space and prevent layout shifts when content changes height. Also adds the missing `overflow-y: auto` to `main` so pages that exceed the viewport height are independently scrollable. [#1087](https://github.com/fsprojects/FSharp.Formatting/issues/1087), [#1088](https://github.com/fsprojects/FSharp.Formatting/pull/1088)
 
 ## 22.0.0-alpha.1 - 2026-03-03
 

docs/content/fsdocs-default.css

@@ -239,6 +239,7 @@ header {
                 list-style: none;
                 padding: var(--spacing-300);
                 overflow-y: auto;
+                scrollbar-gutter: stable;
             }
 
             & input:checked + .menu {
@@ -310,6 +311,8 @@ aside {
 
 main {
     height: calc(100dvh - var(--header-height));
+    overflow-y: auto;
+    scrollbar-gutter: stable;
 }
 
 #content {
@@ -469,6 +472,7 @@ main {
         grid-row: var(--main-menu-grid-row);
         grid-column: var(--main-menu-grid-column);
         overflow-y: auto;
+        scrollbar-gutter: stable;
     }
 
     main {
@@ -1269,6 +1273,7 @@ dialog {
 
     & ul {
         overflow-y: auto;
+        scrollbar-gutter: stable;
         max-height: calc(50vh - var(--spacing-700) - var(--spacing-700));
         list-style: none;
         padding: 0;