Make Clicking in Scrollbars Move By Page

[AE1020]

This changes clicking in the "gutter" area of a scrollbar from
jumping to a position in the file proportional to the click, to
the more standard behavior of "clicking before the slider does a
page up (or left), and clicking after the slider does a page down
(or right)".

The behavior is requested in #43564 (and 101698, 100446, 87273
70267, 62834, 55139...) It would likely make a reasonable
default, since clicking in the source preview right next to the
scroll bar already permits jumping to absolute positions.

However, this PR could also be amended to make it an option.
Documentation of the implementation process is here:

https://ae1020.github.io/vscode-scrollbar-go-by-page/

This PR fixes #43564

[ghost]

All CLA requirements met.

[AE1020]

There are two tests in scrollbarState.test.ts which construct a version of the internal state object with literal values, and calls ScrollbarState.getDesiredPositionFromOffset() function and expect a value back.

This commit changes (only) that function. The new results make sense, and it's certainly possible to just change the existing numbers, if that is acceptable--depending on what exactly these tests were wanting to check in the first place.

Test One - excerpted/annotated (was 32849, is now 33126)

let actual = new ScrollbarState(
    0,  // arrowSize
    14,  // scrollbarSize
    0,  // oppositeScrollbarSize
    339,  // visibleSize
    42423,  // scrollSize
    32787  // scrollPosition
);

assert.equal(actual.getSliderPosition(), 249);
assert.equal(actual.getDesiredScrollPositionFromOffset(259), 32849);  // this changed

Here it registered as a "page down", because the click at 259 is after the slider (249) in "slider/offset units". So the visibleSize was added in scrollPosition units (32787 + 339 = 33126) .

Test Two - excerpted/annotated: (was 32811, now 33126)

let actual = new ScrollbarState(
    12,  // arrowSize
    14,  // scrollbarSize
    0,  // oppositeScrollbarSize
    339,  // visibleSize
    42423,  // scrollSize
    32787  // scrollPosition
 );

assert.equal(actual.getSliderPosition(), 230);
assert.equal(actual.getDesiredScrollPositionFromOffset(240 + 12), 32811);  // this changed

Again, this acts as a "page down", because the click at 252 is after the slider (230) in "slider/offset units". So the visibleSize was added in scrollPosition units (32787 + 339 = 33126).

[alexdima]

This new behaviour needs to be behind a setting. We have very many users who have gotten used to the current behaviour. The current behaviour is intentional, because it allows to click on an overview ruler indicator (like an error) and jump directly to the right location.

[AE1020]

This new behaviour needs to be behind a setting.

Added editor.scrollbar.gutterClickMovesByPage, trying to follow the existing scrollbar pattern for settings (read as options in editorScrollbar.ts).

[AE1020]

@alexdima Please let me know if any further changes are needed, while I still have this set up to work on.

This new behaviour needs to be behind a setting.

I made the setting editor.scrollbar.gutterClickMovesByPage default to false, so that the behavior will have to be explicitly changed. So it should not be disruptive in the near term. The question of whether it makes a better default to use a traditional scroll bar can be revisited after people have experience with the option.

We have very many users who have gotten used to the current behaviour. The current behaviour is intentional, because it allows to click on an overview ruler indicator (like an error) and jump directly to the right location.

As a data point when I described this issue on the phone to a friend, without prompting he offered "Can't you just click on that preview strip that shows a miniature version of the source if you wanted to jump to an absolute position, and have the scroll bar act like a normal scroll bar when you click?" So again, I think the scenario you discuss about jumping to specific highlighted positions should be covered by that method--not changing how scroll bars act.

But again: that can be a separate issue or discussion, as this commit does not change it from how it is today.

[RedDragonWebDesign]

Looks like this PR hasn't had a comment since September and might be stuck. I just want to make a comment showing support for this and hopefully encourage folks to review/approve it. Thank you for your time and effort everyone.

[alexdima]

Thank you!