How to get Scrollbar thumb position?

Question

I'm working on creating a 'dummy' ScrollBar to replace the original Listbox Scrollbar, then I can customize its shape/colors, etc, for this, I need the information about each item index relative to the thumb position.

I'm calling GetScrollbarInfo and SetScrollInfo in a loop that moves the thumb around and get its position:

   While (I <= M) {  
      NumPut(I, SI, 20, "Int") ; nPos  
      DllCall("SetScrollInfo", "Ptr", hWnd, "UInt", 1, "Ptr", &SI, "UInt", 0, "Int")  
      DllCall("GetScrollBarInfo", "Ptr", hWnd, "Int", 0xFFFFFFFB, "Ptr", &SBI)  
      VSB[I, "THT"] := NumGet(SBI, 24, "Int") ; thumb top  
      VSB[I, "THB"] := NumGet(SBI, 28, "Int") ; thumb bottom  
      I++  
   }

However when there are many items in a Listbox, GetScrollBarInfo starts returning a different top position than where it currently is:

TopIndex 100 reports Thumb Top at 33, but its really at 29 as seen in the image below:

Also deducting the bottom - top (41-33) we have 8, why 8 if the thumb height is 17?

Why does this happen? when the are just a few items it does report each position correctly.

OBS:
----
The language in the script below is AutoHotkey I tried to get some help in the AutoHotkey forum but its a very specific question related to WinApi.
I'm adding the tag c++ to see if I can target more people and get any kind of help, so answers in c++ are also welcome.

The relevant code is only inside of the function GetVScrollBarPositions and it's all commented.

Text := ""  
Gui, Color, 0  
Loop, 200  
   Text .= A_Index . "A|"  
Gui, +E0x02000000 +E0x00080000  
Gui, Add, Listbox, w100 r5 +VScroll hWndhWnd, %Text%  
GuiControlGet, P, Pos, %hWnd%  
Gui, Add, Text, xm, ListBox Height: %Ph%  
Gui, Show, w300 h356, Test  
Return  
  
GuiClose:  
ExitApp  
  
F2::  
S := A_TickCount  
VSB := GetVScrollBarPositions(hWnd)  
T := A_TickCount - S  
  
Gui, Result:New, +Owner1  
Gui, Add, Text, xm w250, Range Min:  
Gui, Add, Text, x+0 yp w100, % VSB.Min  
Gui, Add, Text, xm w250, Range Max:  
Gui, Add, Text, x+0 yp w100, % VSB.Max  
Gui, Add, Text, xm w250, Page Size:  
Gui, Add, Text, x+0 yp w100, % VSB.Page  
Gui, Add, Text, xm w250, SB Top:  
Gui, Add, Text, x+0 yp w100, % VSB.SBT  
Gui, Add, Text, xm w250, SB Bottom:  
Gui, Add, Text, x+0 yp w100, % VSB.SBB  
Gui, Add, Text, xm w250, Arrow Height:  
Gui, Add, Text, x+0 yp w100, % VSB.ABH  
Gui, Add, ListView, xm w350 r10, Item Index|Thumb Top|Thumb Bottom  
I := VSB.Min  
M := VSB.Max  
While (I <= M) {  
   LV_Add("", I, VSB[I, "THT"], VSB[I, "THB"])  
   I++  
}  
Gui, Show, , Vertical Scroll Bar Info (%T% ms)  
Return  
  
  
  
GetVScrollBarPositions(hWnd) {  
  
   VarSetCapacity(SI, 28, 0)   ; SCROLLINFO structure  
   NumPut(28, SI, 0, "UInt")   ; cbSize  
   NumPut(0x17, SI, 4, "UInt") ; fMask = SIF_ALL  
   If !DllCall("GetScrollInfo", "Ptr", hWnd, "Int", 1, "Ptr", &SI)  
      Return False  
  
   VSB := {}  
   VSB.Min := NumGet(SI, 8, "Int")   ; range min  
   VSB.Max := NumGet(SI, 12, "Int")  ; range max  
   VSB.Page := NumGet(SI, 16, "Int") ; page size  
   Pos := NumGet(SI, 20, "Int")      ; current pos  
  
   VarSetCapacity(SBI, 60, 0)  
   NumPut(60, SBI, 0, "Int")  
   DllCall("GetScrollBarInfo", "Ptr", hWnd, "Int", 0xFFFFFFFB, "Ptr", &SBI)  
   VSB.SBT := NumGet(SBI, 8, "Int")  ; scroll bar top  
   VSB.SBB := NumGet(SBI, 16, "Int") ; scroll bar bottom  
   VSB.ABH := NumGet(SBI, 20, "Int") ; arrow button height  
     
   NumPut(4, SI,  4, "Int") ; fMask = SIF_POS  
   I := VSB.Min  
   M := VSB.Max  
  
   While (I <= M) {  
      NumPut(I, SI, 20, "Int") ; nPos  
      DllCall("SetScrollInfo", "Ptr", hWnd, "UInt", 1, "Ptr", &SI, "UInt", 0, "Int")  
      DllCall("GetScrollBarInfo", "Ptr", hWnd, "Int", 0xFFFFFFFB, "Ptr", &SBI)  
      VSB[I, "THT"] := NumGet(SBI, 24, "Int") ; thumb top  
      VSB[I, "THB"] := NumGet(SBI, 28, "Int") ; thumb bottom  
      I++  
   }  
     
   NumPut(Pos, SI, 20, "Int") ; restore the original pos  
   DllCall("SetScrollInfo", "Ptr", hWnd, "UInt", 1, "Ptr", &SI, "UInt", 1, "Int")  
  
   Return VSB  
}

Answer

Hello,
Welcome to Microsoft Q&A!

For the convenience of viewing the pictures, I will first post my test process here.

my calculation formula (floating point calculations):
BTW, The number of rows in listbox starts from 0.

ListRowPos = ( ThumbPos - ArrowHeight ) / ( ScrollArea ) * TotalRows;

**Test 1: **
**A total of 800 pieces of data, set the maximum range of the scroll bar to 800 **
Because the floating point number is rounded after calculation, there will be a deviation of one line.

**Test 2: **
**A total of 800 pieces of data, set the maximum range of the scroll bar to 10 **
The slider slides smoothly, but the list slides very stiffly.

I hope my formula is helpful to you. My project is just a simple beta version, and the constant variables in it are all directly assigned. If you need, I can share the code in the txt document.

Thank you.

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How to get Scrollbar thumb position?

1 answer