fix pages/grid after switching single page step (#3957)

* fix switching single page step

* Update CHANGELOG.md

src/pdfviewer/PDFDocument.cpp

@@ -2265,7 +2265,8 @@ void PDFWidget::setSinglePageStep(bool step)
 		return;
 	singlePageStep = step;
 	getScrollArea()->goToPage(realPageIndex);
-    delayedUpdate();
+	reloadPage();
+	getScrollArea()->updateScrollBars();
 }
 
 void PDFWidget::goFirst()

utilities/manual/source/CHANGELOG.md

@@ -12,6 +12,7 @@
 - fix pdf-viewer's scrollbar with Fit to Width/Window and changing Continuous mode [#3928](https://github.com/texstudio-org/texstudio/pull/3928)
 - fix pdf-viewer's Custom Grid dialog not preset with current Grid settings in Continuous mode [#3929](https://github.com/texstudio-org/texstudio/pull/3929)
 - fix pfd-viewer's page display in non continuous mode [3952](https://github.com/texstudio-org/texstudio/pull/3952)
+- fix pdf-viewer has a small issue when activating single page step [3957](https://github.com/texstudio-org/texstudio/pull/3957)
 - add maximize button to Packages Help (Texdoc) dialog [#3911](https://github.com/texstudio-org/texstudio/pull/3911)
 - fix option 'all packages' no longer checked in Packages Help with no tex documents opened [#3917](https://github.com/texstudio-org/texstudio/pull/3917)
 - when context menu of a package name is used to open the Packages Help dialog then preset search filter with the name [#3918](https://github.com/texstudio-org/texstudio/pull/3918)