When you click a link to deleted (or just nonexistent) revision on the web, MediaWiki:Difference-missing-revision is shown which explains what happened.

• Example: https://en.wikipedia.org/wiki/Special:Diff/898964501898964501

But when you click the same link in mobile view, it shows unhelpful plain error message:
Bad revision. The ID for the revision you requested does not exist.

• https://en.m.wikipedia.org/wiki/Special:MobileDiff/898964501898964501

Developer notes

The problem is located in SpecialMobileDiff::executeWhenAvailable() function. When $rev is null (no revision found), instead of normal flow system calls
wfHttpError( 404, title, text ); and returns early. wfHttpError function returns the error in oldchool/ugly way without any layout (as it says, it's simple http errror.

Code:

	public function executeWhenAvailable( $par ) {
                [...]
		$revisions = $this->getRevisionsToCompare( explode( '...', $par, 2 ) );
		list( $prev, $rev ) = $revisions;

		if ( $rev === null ) {
			$this->executeBadQuery();
			return false;
		}
                [...]
         }

        /**
	 * Generate a 404 Error message, that revisions can not be found
	 */
	public function executeBadQuery() {
		wfHttpError( 404, $this->msg( 'mobile-frontend-diffview-404-title' )->text(),
			$this->msg( 'mobile-frontend-diffview-404-desc' )->text() );
	}

Instead please use $this->showPageNotFound(). TheSpecialMobileDiff should override errorNotFoundDescriptionMsg and errorNotFoundTitleMsg properties.

Acceptance criteria

• When $revision is not found system outputs nice error page
• wfHttpError() is not used

QA

Visit https://en.m.wikipedia.beta.wmflabs.org/wiki/Special:MobileDiff/898964501898964501

This should show a friendly error message in the Minerva skin. It should not show a blank white page with the text "The ID for the revision you requested does not exist."

QA results

T224430#5722992