Prevent ScrollArea contents from exceeding the container size

[DouglasDwyer]

When a ScrollArea is added to a Ui or its contents change dynamically, the contents will briefly escape the container. This occurs because ScrollArea internally maintains content_is_too_large flags, from which it determines when to clip. The content_is_too_large flags are calculated after painting, so they always lag one frame behind. This can lead to flickering.

To fix this, I have changed the ScrollArea so that it always clips scrollable content. I believe that this should fix things without negatively impacting other behavior. To see this, consider how ScrollArea calculates the content_is_too_large flag:

// This calculates a new inner rect, after painting, from the initial clip rect
let inner_rect = {
  // At this point this is the available size for the inner rect.
  let mut inner_size = inner_rect.size();
  
  for d in 0..2 {
      inner_size[d] = match (scroll_enabled[d], auto_shrink[d]) {
          (true, true) => inner_size[d].min(content_size[d]), // shrink scroll area if content is small
          (true, false) => inner_size[d], // let scroll area be larger than content; fill with blank space
          (false, true) => content_size[d], // Follow the content (expand/contract to fit it).
          (false, false) => inner_size[d].max(content_size[d]), // Expand to fit content
      };
  }
  
  Rect::from_min_size(inner_rect.min, inner_size)
};

let outer_rect = Rect::from_min_size(inner_rect.min, inner_rect.size() + current_bar_use);

let content_is_too_large = Vec2b::new(
  scroll_enabled[0] && inner_rect.width() < content_size.x,
  scroll_enabled[1] && inner_rect.height() < content_size.y,
);

If scroll_enabled[d] == true, then the actual inner_rect (which is calculated after painting contents) will always be smaller than the original inner_rect. Hence, it is safe to unconditionally clip the contents to inner_rect whenever scroll_enabled[d] == true.

• Closes ScrollArea contents briefly escape it when it becomes larger #4742
• I have followed the instructions in the PR template