Bug 565883

Summary:

[GTK] Scrolling in Canvas is different than in Table

Product:

[Eclipse Project] Platform

Reporter:

Thomas Singer <ts-swt>

Component:

SWT

Assignee:

Platform-SWT-Inbox <platform-swt-inbox>

Status:

NEW ---

QA Contact:

Severity:

normal

Priority:

CC:

alexandr.miloslavskiy, sdamrong, ts-swt

Version:

4.15

Target Milestone:

---

Hardware:

OS:

Linux

See Also:

https://bugs.eclipse.org/bugs/show_bug.cgi?id=565426

Whiteboard:

Attachments:

Description	Flags
Snippet to reproduce	none
After scrolling 2 steps	none
Snippet with large scroll steps in Canvas	none

Description Thomas Singer

2020-08-07 05:44:31 EDT

Please launch the attached snippet on Linux and use the mouse wheel to scroll. In the left Canvas scrolling appears to be much slower (sometimes one mouse wheel step does not result in any scrolling at all) then in the right Table (often one mouse wheel step results in 2 scrolled rows).

Comment 1 Thomas Singer

2020-08-07 05:45:15 EDT

Created attachment 283811 [details]
Snippet to reproduce

Comment 2 Thomas Singer

2020-08-07 05:46:50 EDT

Created attachment 283812 [details]
After scrolling 2 steps

Comment 3 Thomas Singer

2020-08-07 06:17:37 EDT

Created attachment 283813 [details]
Snippet with large scroll steps in Canvas

Comment 4 Thomas Singer

2020-08-07 06:18:50 EDT

In the "Snippet with large scroll steps in Canvas" it scrolls 9 rows (Canvas) vs. 2 (Table) per mouse wheel step.

Comment 5 Soraphol (Paul) Damrongpiriyapong

2020-08-21 14:33:53 EDT

Hello, I have tested the snippets and reproduce the same results. However, I'm not sure what the problem is here. Since the canvas scrollbar is having custom values set, it will be different than the Table scroll speed.

Comment 6 Thomas Singer

2020-08-24 03:43:10 EDT

(In reply to Soraphol (Paul) Damrongpiriyapong from comment #5)
> Since the canvas scrollbar is having
> custom values set, it will be different than the Table scroll speed.

Try to implement a table-like control (which we did because Table was too slow on some platforms to handle large amounts of data). The user will expect to see the same scroll effect on a Table or custom control. On Windows it works as expected, but not on Linux.

Comment 7 Alexandr Miloslavskiy

2020-09-21 11:18:46 EDT

I found that GTK table behaves the following way (in GTK terms):
1) Scrolling speed is calculated from verticalBar.setThumb()
2) GTK's Table passes Table height to verticalBar.setThumb()
3) The final formula is for pixels scrolled per wheel step:
   min(tableHeight/2, pow(tableHeight, 2/3))

Note that on Windows, scrolling speed does not depend on tableHeight.

So in order to mimic the behavior of GTK, one should do the following:
1) Set max of verticalBar to total pixel height of contents
2) verticalBar.setThumb(canvas.getSize().y)
3) Consider 'verticalBar.getSelection()' to be measured in pixels.

Note that measuring in anything but pixels is hard because GTK uses pow(,2/3) and SWT discards fractional part of 'verticalBar.getSelection()'.

References in GTK code:
* get_scroll_unit() in gtkscrolledwindow.c
* gtk_adjustment_configure() in gtk_tree_view_size_allocate()