Fix scrolling on Wayland

[amolenaar]

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Documentation content changes

What is the current behavior?

Wayland is using smooth scrolling. However, in gaphor the scrollbar jumps when the cursor hits an item.

Issue Number: #480

What is the new behavior?

Scrolling works smooth on Wayland. In addition, while scrolling now only a redraw is done, not a full update.

Does this PR introduce a breaking change?

• Yes
• No

Other information

[sourcery-ai]

Sourcery Code Quality Report

❌  Merging this PR will decrease code quality in the affected files by 0.55%.

Quality metrics	Before	After	Change
Complexity	3.02 ⭐	3.46 ⭐	0.44 👎
Method Length	48.76 ⭐	49.38 ⭐	0.62 👎
Working memory	4.97 ⭐	4.96 ⭐	-0.01 👍
Quality	78.76% ⭐	78.21% ⭐	-0.55% 👎

Other metrics	Before	After	Change
Lines	714	707	-7

Changed files	Quality Before	Quality After	Quality Change
gaphas/view/gtkview.py	76.57% ⭐	76.42% ⭐	-0.15% 👎
gaphas/view/scrolling.py	72.78% 🙂	68.57% 🙂	-4.21% 👎
tests/conftest.py	89.12% ⭐	88.88% ⭐	-0.24% 👎
tests/test_tool_scroll.py	89.13% ⭐	89.13% ⭐	0.00%
tests/test_view.py	82.86% ⭐	85.20% ⭐	2.34% 👍

Here are some functions in these files that still need a tune-up:

File	Function	Complexity	Length	Working Memory	Quality	Recommendation
gaphas/view/gtkview.py	GtkView.update_back_buffer	15 🙂	235 ⛔	10 😞	42.03% 😞	Try splitting into smaller methods. Extract out complex expressions
gaphas/view/scrolling.py	Scrolling.set_property	17 🙂	150 😞	10 😞	47.26% 😞	Try splitting into smaller methods. Extract out complex expressions
gaphas/view/gtkview.py	GtkView.__init__	6 ⭐	226 ⛔		51.44% 🙂	Try splitting into smaller methods
gaphas/view/scrolling.py	Scrolling.update_adjustments	2 ⭐	167 😞	6 ⭐	68.30% 🙂	Try splitting into smaller methods
tests/test_view.py	test_scroll_adjustments_signal	0 ⭐	125 😞	3 ⭐	82.70% ⭐	Try splitting into smaller methods

Legend and Explanation

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• 🙂 good
• 😞 poor
• ⛔ very poor

The 👍 and 👎 indicate whether the quality has improved or gotten worse with this pull request.

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