Add dry run feature for anchor calculation. (#1) (#1856)

* Add dry run feature for anchor calculation. (#1)

This PR resolves issue: #1852
We introduce dry_run flag which makes component invisible until we do second pass of rendering - which allows us to properly calculate position for anchor. (This removes rapid flicker when new window is drawn for the first time).

* Change naming convention and add description

egui/src/containers/area.rs

@@ -172,6 +172,12 @@ pub(crate) struct Prepared {
  pub(crate) movable: bool,
  enabled: bool,
  drag_bounds: Option<Rect>,
+ /// Set the first frame of new windows with anchors.
+ ///
+ /// This is so that we use the first frame to calculate the window size,
+ /// and then can correctly position the window the next frame,
+ /// without having one frame where the window is positioned in the wrong place.
+ temporarily_invisible: bool,
 }
 
 impl Area {
@@ -214,11 +220,11 @@ impl Area {
  });
  state.pos = new_pos.unwrap_or(state.pos);
  state.interactable = interactable;
+ let mut temporarily_invisible = false;
 
  if let Some((anchor, offset)) = anchor {
  if is_new {
- // unknown size
- ctx.request_repaint();
+ temporarily_invisible = true;
  } else {
  let screen = ctx.available_rect();
  state.pos = anchor.align_size_within_rect(state.size, screen).min + offset;
@@ -233,6 +239,7 @@ impl Area {
  movable,
  enabled,
  drag_bounds,
+ temporarily_invisible,
  }
  }
 
@@ -314,7 +321,7 @@ impl Prepared {
  clip_rect,
  );
  ui.set_enabled(self.enabled);
-
+ ui.set_visible(!self.temporarily_invisible);
  ui
  }
 
@@ -326,6 +333,7 @@ impl Prepared {
  movable,
  enabled,
  drag_bounds,
+ temporarily_invisible: _,
  } = self;
 
  state.size = content_ui.min_rect().size();