Fix iOS keyboard glitch when switching between topic and content inputs

Disable scribbleEnabled on both topic and content TextFields to prevent
the keyboard from briefly sliding out and back in when switching focus
between the two inputs on iOS.

This is a workaround for an upstream Flutter bug in the iOS keyboard
lifecycle management. The trade-off is that Apple Pencil Scribble
handwriting input is disabled for these fields, which is acceptable
since most users compose with the keyboard.

Fixes: #1974

Author: Kushagra2024-code

lib/widgets/compose_box.dart

@@ -606,6 +606,10 @@ class _ContentInput extends StatelessWidget {
               minLines: 2,
               maxLines: null,
               textCapitalization: TextCapitalization.sentences,
+              // Disable scribble to maintain keyboard continuity on iOS
+              // when switching between topic and content inputs.
+              // See: https://github.com/zulip/zulip-flutter/issues/1974
+              scribbleEnabled: false,
               decoration: InputDecoration(
                 // This padding ensures that the user can always scroll long
                 // content entirely out of the top or bottom shadow if desired.
@@ -867,7 +871,11 @@ class _TopicInputState extends State<_TopicInput> {
           focusNode: widget.controller.topicFocusNode,
           textInputAction: TextInputAction.next,
           style: topicTextStyle,
-          decoration: decoration)));
+          decoration: decoration,
+          // Disable scribble to maintain keyboard continuity on iOS
+          // when switching between topic and content inputs.
+          // See: https://github.com/zulip/zulip-flutter/issues/1974
+          scribbleEnabled: false)));
   }
 }