Disabling smooth scrolling on wordpad

I’ve always been annoyed by smooth scrolling, and if you’re reading this blog post, you’re probably annoyed too. Wordpad on windows has smooth scrolling on, and a quick browse through the settings turns up no option to disable this. Time to debug!

A quick browse through the modules list (windbg’s lm) shows up something interesting, msftedit.dll, which a quick googling shows that it allows applications to embed rich text text-boxes. This is the dll for the latest version of the rich text edit control, v4.1. Previous versions include riched20.dll and riched32.dll.

Since that is basically what wordpad is doing, could wordpad simply have embedded a rich text box in it’s window? Breaking on msftedit!RichEditWndProc shows that this is indeed the case, where scrolling the text box breaks on this routine, with WM_MOUSEWHEEL as the message parameter.

Rich edit boxes are not only used under wordpad, but all over the place. Examples include tortoisegit and windbg.

Digging a bit further, we see that the routine that starts the scrolling is the following:

0051ee3c 5ec19ed8 Msftedit!CDisplay::SmoothVScroll
0051ee60 5ec0d6de Msftedit!CMagellan::MagellanRollScroll+0x98
0051eea8 5ebb8f66 Msftedit!CTxtEdit::HandleMouseWheel+0xee
0051f028 5ebe38e5 Msftedit!CTxtEdit::TxSendMessage+0xe15
0051f918 76de62fa Msftedit!RichEditWndProc+0xa31

Fortunately for us, SmoothVScroll is open sourced in windows CE, which helps in analysis. But going through each of the methods starting from HandleMouseWheel, we see that there are a couple of places the call to SmoothVScroll can be skipped, depending on some flags. Unfortunately none give us the desired behaviour. Some flags control zooming, and others disable scrolling entirely.

The relevant header file defines SmoothVScroll ‘s as:

VOID    SmoothVScroll ( int direction, WORD cLines, int speedNum, int speedDenom, BOOL fMouseRoller );

We can see some speed arguments here. Patching speedNum to 1 and speedDenom to 50, to give a speed of 1/50 instead of the default 3/1, does give us instant scrolling behavour, But SmoothVScroll also has a kind of acceleration effect, where if we scroll longer using the mouse wheel the scrolling speed will accelerate. This can be rather annoying while editing text.

There is a way to skip the call to SmoothVScroll entirely, and without going into internal functions, by hooking RichEditWndProc and sending WM_VSCROLL when WM_MOUSEWHEEL is received. The former occurs when we press the up and down buttons on the scrollbar.

We can also scroll faster by sending it more than once.

I implemented the above hooks in rich edit scroll, which uses minhook, SetWindowsHookEx and friends. Run install.bat to run it on startup, and smooth scrolling will be gotten rid of for good.

Rich edit scroll allows configuring which applications to hook as well as scrolling speed. Hooking is opt-in due to exe protectors often checking whether LoadLibrary is hooked, which is sometimes necessary, in the case of windbg. That can cause them to stop working.