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* Part of https://github.com/emilk/egui/issues/4535
* Closes https://github.com/emilk/egui/issues/3974
This adds a special `sizing_pass` mode to `Ui`, in which we have no
centered or justified layouts, and everything is hidden. This is used by
`Area` to use the first frame to measure the size of its contents so
that it can then set the perfectly correct size the subsequent frames.
For menus, where buttons are justified (span the full width), this
finally the problem of auto-sizing. Before you would have to pick a
width manually, and all buttons would expand to that width. If it was
too wide, it looked weird. If it was too narrow, text would wrap. Now
all menus are exactly the width they need to be. By default menus will
wrap at `Spacing::menu_width`.
This affects all situations when you have something that should be as
small as possible, but still span the full width/height of the parent.
For instance: the `egui::Separator` widget now checks the
`ui.is_sizing_pass` flag before deciding on a size. In the sizing pass a
horizontal separator is always 0 wide, and only in subsequent passes
will it span the full width.
These three types currently have a `Debug` implementation that only ever
prints one decimal point. Sometimes it is useful to see more of the
number, or otherwise have specific formatting.
Add `Display` implementations that pass the format specification to the
member `f32`s for an easier way to control what is shown when debugging.
This allows doing e.g. `ui.label(format!("{:.4}", rect * scale))` which
currently prints zeroes if scale is small.
* Functions that take Stroke were updated to take Into<Stroke> to make
them consistent with other Into<Stroke> parameters.
* Vec2 implements DivAssign<f32>, to make it consistent with already
implementing MulAssign<f32> and Div<f32>.
* Vec2::angled() uses sin_cos() rather than an individual sin() and
cos() call for an immeasurable but hypothetical performance improvement.
* Disable the lock_reentry_single_thread() mutex test. Lock()ing twice
on the same thread is not guaranteed to panic.
* Closes <https://github.com/emilk/egui/issues/3419>.
* Move scroll bar spacing settings to a `struct ScrollSpacing`
* Add a demo for changing scroll bar appearance
* Add setting for ScrollBarVisibility in demo
* Add `#[inline]` to a `ScrollArea` builder methods
* Refactor how scroll bar show/hide is computed
* Add support for floating scroll bars
* Tweak color and opacity of the scroll handle
* Allow allocating a fixed size even for floating scroll bars
* Add three pre-sets of scroll bars: solid, thin, floating
* Use floating scroll bars as the default
* Fix id-clash with bidir scroll areas
* Improve demo
* Fix doclink
* Remove reset button from demo
* Fix doclinks
* Fix visual artifact with thin rounded rectangles
* Fix doclink
* typos
