Diagnostic controller improvement

Summary:
The diagnostics controller is not the best crafted UIViewController. It has many things that should be improved.

- This diff just sets the background of the root view to white. At least, that will ensure that its text content is always shown if the screen background happens to be black (same as font colour).
- Correct offset calculation, used for scrolling. Effectively, only set it if the content to be displayed no longer fits in the scrollview viewport.

Reviewed By: LukeDefeo

Differential Revision: D41876904

fbshipit-source-id: e2a89d8f6001e5b626c8df1d0832e77783999b81

iOS/FlipperKit/FlipperDiagnosticsViewController.m

@@ -37,6 +37,7 @@ static NSString* const kSKCellIdentifier =
   cell.textLabel.font = [UIFont fontWithName:@"Arial" size:10];
   cell.textLabel.text = [self.elements[row][@"state"]
       stringByAppendingString:self.elements[row][@"name"]];
+
   return cell;
 }
 
@@ -52,12 +53,19 @@ static NSString* const kSKCellIdentifier =
 - (void)viewDidLoad {
   [super viewDidLoad];
 
+  self.view.backgroundColor = [UIColor whiteColor];
+
+  self.stateTable = [[UITableView alloc]
+      initWithFrame:CGRectMake(
+                        0, 0, self.view.bounds.size.width, STATE_VIEW_HEIGHT)];
+
   self.scrollView = [[UIScrollView alloc]
       initWithFrame:CGRectMake(
                         0,
                         STATE_VIEW_HEIGHT,
                         self.view.frame.size.width,
                         self.view.frame.size.height - 100 - STATE_VIEW_HEIGHT)];
+
   self.logLabel =
       [[UILabel alloc] initWithFrame:CGRectMake(
                                          0,
@@ -66,11 +74,9 @@ static NSString* const kSKCellIdentifier =
                                          self.scrollView.frame.size.height)];
   self.logLabel.numberOfLines = 0;
   self.logLabel.font = [UIFont systemFontOfSize:10.0f];
+
   [self.scrollView addSubview:self.logLabel];
 
-  self.stateTable = [[UITableView alloc]
-      initWithFrame:CGRectMake(
-                        0, 0, self.view.bounds.size.width, STATE_VIEW_HEIGHT)];
   [self.stateTable registerClass:[UITableViewCell class]
           forCellReuseIdentifier:kSKCellIdentifier];
   self.stateTable.rowHeight = 14;
@@ -104,10 +110,11 @@ static NSString* const kSKCellIdentifier =
   [self.logLabel sizeToFit];
   self.scrollView.contentSize = self.logLabel.frame.size;
 
-  // Scroll to bottom
+  CGFloat y = 0;
-  CGPoint bottomOffset = CGPointMake(
+  if (self.scrollView.contentSize.height > self.scrollView.bounds.size.height) {
-      0,
+    y = self.scrollView.contentSize.height - self.scrollView.bounds.size.height;
-      self.scrollView.contentSize.height - self.scrollView.bounds.size.height);
+  }
+  CGPoint bottomOffset = CGPointMake(0, y);
   [self.scrollView setContentOffset:bottomOffset animated:YES];
 }