Mac: constrainFrameRect:toScreen: rework

When the screen passed to `constrainFrameRect:toScreen:` is nil,
AppKit does some gymnastics to select a screen, only defaulting
to the main screen if there's no result. We don't have access
to these calculations in an override, so we can't use the
screen's frame for heuristics when deciding whether to allow
the constrain to happen or not.

This change removes the previous logic in favor of trying
to test specifically for the large vertical jumps that cause
glitches when detaching tabs.

Bug: 1102925
Change-Id: Ic5653d5fd867da155605cd986ec80c5bb4afd755
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2298056Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Leonard Grey <lgrey@chromium.org>
Cr-Commit-Position: refs/heads/master@{#789503}

components/remote_cocoa/app_shim/browser_native_widget_window_mac.mm

@@ -67,23 +67,19 @@
 
 // Prevent detached tabs from glitching when the window is partially offscreen.
 // See https://crbug.com/1095717 for details.
+// This is easy to get wrong so scope very tightly to only disallow large
+// vertical jumps.
 - (NSRect)constrainFrameRect:(NSRect)rect toScreen:(NSScreen*)screen {
-  if (!screen)
-    screen = [NSScreen mainScreen];
-  // A small margin so that constraining kicks in before we're *all the way*
-  // to the edge of the screen.
-  static constexpr CGFloat kThreshold = 80;
-  NSRect screenFrame = [screen frame];
-  // Adjust if either the entire frame is offscreen, or the toolbar is
-  // cut off at the top.
-  if (NSMaxY(rect) - kThreshold < NSMinY(screenFrame) ||  // Below the screen.
-      NSMaxX(rect) - kThreshold < NSMinX(screenFrame) ||  // Left of the screen.
-      NSMinX(rect) + kThreshold >
-          NSMaxX(screenFrame) ||  // Right of the screen.
-      NSMaxY(rect) + kThreshold > NSMaxY(screenFrame)) {  // Top above screen.
-    return [super constrainFrameRect:rect toScreen:screen];
-  }
-  return rect;
+  NSRect proposed = [super constrainFrameRect:rect toScreen:screen];
+  // This boils down to: use the small threshold when we're not avoiding a
+  // Dock on the bottom, and the big threshold otherwise.
+  static constexpr CGFloat kBigThreshold = 200;
+  static constexpr CGFloat kSmallThreshold = 50;
+  const CGFloat yDelta = NSMaxY(proposed) - NSMaxY(rect);
+  if (yDelta > kBigThreshold ||
+      (yDelta > kSmallThreshold && NSMinY(proposed) == 0))
+    return rect;
+  return proposed;
 }
 
 // NSWindow (PrivateAPI) overrides.