Resizable Editor: Unintended vertical scrollbar

[t-hamano]

Description

In the resizable editor, if there is little content, the vertical scrollbar is not necessary, but it seems to always be displayed.

After investigating this issue, I found that it was caused by the top border added by #66041.

Step-by-step reproduction instructions

• Open the site editor.
• Open the pattern or template part editor.

Screenshots, screen recording, code snippet

If I temporarily disable the top border this issue doesn't occur:

Environment info

No response

Please confirm that you have searched existing issues in the repo.

• Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

• Yes

[andrewserong]

Oh, interesting, good catch!

It seems that the resize observer gives a rounded integer instead of the precise number of the actual size, and this border rule uses sub-pixels, which means we get a height of a resizable editor that is just not tall enough for the content:

gutenberg/packages/compose/src/hooks/use-resize-observer/legacy/index.tsx

Line 65 in a90e35b

const [ width, height ] = entrySize.map( ( d ) => Math.round( d ) );

(I believe we're using the legacy implementation because useResizeObserver is called without passing in a callback).

So, I'm not sure if this is the right way to fix it, but it seems like we could always add 1 to the height in the visual editor to ensure that the height accounts for the border. Would that be a terrible approach?

diff --git a/packages/editor/src/components/visual-editor/index.js b/packages/editor/src/components/visual-editor/index.js
index 8ad9ce7fc97..fe261fed4b7 100644
--- a/packages/editor/src/components/visual-editor/index.js
+++ b/packages/editor/src/components/visual-editor/index.js
@@ -380,15 +380,17 @@ function VisualEditor( {
 					'is-iframed': shouldIframe,
 				}
 			) }
 		>
 			<ResizableEditor
 				enableResizing={ enableResizing }
 				height={
-					sizes.height && ! forceFullHeight ? sizes.height : '100%'
+					sizes.height && ! forceFullHeight
+						? sizes.height + 1
+						: '100%'
 				}
 			>
 				<BlockCanvas
 					shouldIframe={ shouldIframe }
 					contentRef={ contentRef }
 					styles={ iframeStyles }
 					height="100%"

I can throw that up in a quick PR if it helps.

[andrewserong]

Opened a PR in #66301 — happy to close it out, though, if there's a better fix!