types: Relocate some $FlowFixMes and correct their TODOs.

As noted in the code comments, one mistake we were making (not seen
until now) was passing the wrong type parameter to `React$Ref`; we
should be passing a `React$ElementRef` type [1].

When we do that, the existing `$FlowFixMe`s (the ones with comments
about RN v0.63) are marked as unused, as we'd expect. But
unfortunately, at that point, `.current` is typed as
`any(implicit)`, which we definitely don't want. And that doesn't
seem to get fixed by upgrading to RN v0.63, so, remove the comments
about that new version. Greg suggests this may be caused by bugs in
Flow's special support for React [2].

So, mention our new knowledge that we need to be using
`React$ElementRef`, but retain some fixmes and comments alerting us
to the fact that `.current` is untyped and that we need to be
careful about using it.

[1] zulip#4278 (comment)
[2] zulip#4420 (comment)

src/common/Input.js

@@ -11,7 +11,12 @@ export type Props = $ReadOnly<{|
   ...React$ElementConfig<typeof TextInput>,
   placeholder: LocalizableText,
   onChangeText?: (text: string) => void,
-  textInputRef?: React$Ref<typeof TextInput>,
+
+  // We should replace the fixme with
+  // `React$ElementRef<typeof TextInput>` when we can. Currently, that
+  // would make `.current` be `any(implicit)`, which we don't want;
+  // this is probably down to bugs in Flow's special support for React.
+  textInputRef?: React$Ref<$FlowFixMe>,
 
   _: GetText,
 |}>;

src/common/InputWithClearButton.js

@@ -1,6 +1,6 @@
 /* @flow strict-local */
 import React, { PureComponent } from 'react';
-import { View, TextInput } from 'react-native';
+import { View } from 'react-native';
 
 import Input from './Input';
 import type { Props as InputProps } from './Input';
@@ -32,7 +32,11 @@ export default class InputWithClearButton extends PureComponent<Props, State> {
     canBeCleared: false,
     text: '',
   };
-  textInputRef = React.createRef<typeof TextInput>();
+  // We should replace the fixme with
+  // `React$ElementRef<typeof TextInput>` when we can. Currently, that
+  // would make `.current` be `any(implicit)`, which we don't want;
+  // this is probably down to bugs in Flow's special support for React.
+  textInputRef = React.createRef<$FlowFixMe>();
 
   handleChangeText = (text: string) => {
     this.setState({
@@ -47,9 +51,7 @@ export default class InputWithClearButton extends PureComponent<Props, State> {
   handleClear = () => {
     this.handleChangeText('');
     if (this.textInputRef.current) {
-      // Should be fixed in RN v0.63 (#4245); see
-      // https://github.com/zulip/zulip-mobile/issues/4245#issuecomment-695104351.
-      // $FlowFixMe
+      // `.current` is not type-checked; see definition.
       this.textInputRef.current.clear();
     }
   };

src/common/SmartUrlInput.js

@@ -69,15 +69,19 @@ export default class SmartUrlInput extends PureComponent<Props, State> {
   state = {
     value: '',
   };
-  textInputRef = React.createRef<typeof TextInput>();
+
+  // We should replace the fixme with
+  // `React$ElementRef<typeof TextInput>` when we can. Currently, that
+  // would make `.current` be `any(implicit)`, which we don't want;
+  // this is probably down to bugs in Flow's special support for React.
+  textInputRef = React.createRef<$FlowFixMe>();
+
   unsubscribeFocusListener: () => void;
 
   componentDidMount() {
     this.unsubscribeFocusListener = this.props.navigation.addListener('focus', () => {
       if (this.textInputRef.current) {
-        // Should be fixed in RN v0.63 (#4245); see
-        // https://github.com/zulip/zulip-mobile/issues/4245#issuecomment-695104351.
-        // $FlowFixMe
+        // `.current` is not type-checked; see definition.
         this.textInputRef.current.focus();
       }
     });
@@ -99,13 +103,11 @@ export default class SmartUrlInput extends PureComponent<Props, State> {
   urlPress = () => {
     const { textInputRef } = this;
     if (textInputRef.current) {
-      // Should be fixed in RN v0.63 (#4245); see
-      // https://github.com/zulip/zulip-mobile/issues/4245#issuecomment-695104351.
-      // $FlowFixMe
+      // `.current` is not type-checked; see definition.
       textInputRef.current.blur();
       setTimeout(() => {
         if (textInputRef.current) {
-          // $FlowFixMe - same as above
+          // `.current` is not type-checked; see definition.
           textInputRef.current.focus();
         }
       }, 100);

src/compose/ComposeBox.js

@@ -1,6 +1,6 @@
 /* @flow strict-local */
 import React, { PureComponent } from 'react';
-import { Platform, View, TextInput, findNodeHandle } from 'react-native';
+import { Platform, View, findNodeHandle } from 'react-native';
 import type { LayoutEvent } from 'react-native/Libraries/Types/CoreEventTypes';
 import TextInputReset from 'react-native-text-input-reset';
 import { type EdgeInsets } from 'react-native-safe-area-context';
@@ -114,9 +114,7 @@ const updateTextInput = (textInput, text) => {
     return;
   }
 
-  // Should be fixed in RN v0.63 (#4245); see
-  // https://github.com/zulip/zulip-mobile/issues/4245#issuecomment-695104351.
-  // $FlowFixMe
+  // `textInput` is untyped; see definition.
   textInput.setNativeProps({ text });
 
   if (text.length === 0 && TextInputReset) {
@@ -130,8 +128,12 @@ class ComposeBox extends PureComponent<Props, State> {
   static contextType = ThemeContext;
   context: ThemeData;
 
-  messageInputRef = React.createRef<typeof TextInput>();
-  topicInputRef = React.createRef<typeof TextInput>();
+  // We should replace the fixme with
+  // `React$ElementRef<typeof TextInput>` when we can. Currently, that
+  // would make `.current` be `any(implicit)`, which we don't want;
+  // this is probably down to bugs in Flow's special support for React.
+  messageInputRef = React.createRef<$FlowFixMe>();
+  topicInputRef = React.createRef<$FlowFixMe>();
 
   // TODO: Type-check this, once we've adjusted our `react-redux`
   // wrapper to do the right thing. It should be
@@ -349,9 +351,7 @@ class ComposeBox extends PureComponent<Props, State> {
     }
     completeEditMessage();
     if (this.messageInputRef.current !== null) {
-      // Should be fixed in RN v0.63 (#4245); see
-      // https://github.com/zulip/zulip-mobile/issues/4245#issuecomment-695104351.
-      // $FlowFixMe
+      // `.current` is not type-checked; see definition.
       this.messageInputRef.current.blur();
     }
   };
@@ -366,9 +366,7 @@ class ComposeBox extends PureComponent<Props, State> {
       this.setMessageInputValue(message);
       this.setTopicInputValue(topic);
       if (this.messageInputRef.current !== null) {
-        // Should be fixed in RN v0.63 (#4245); see
-        // https://github.com/zulip/zulip-mobile/issues/4245#issuecomment-695104351.
-        // $FlowFixMe
+        // `.current` is not type-checked; see definition.
         this.messageInputRef.current.focus();
       }
     }