Allow new file and edit file preview if it has editable extension (go-gitea#23624)

Close go-gitea#23579 
Inspired by
[idea](go-gitea#23579 (comment))
from @brechtvl
In this PR, the behavior is when extension switches from writatble to
not, preview will hide, and vice versa.

demo:

https://user-images.githubusercontent.com/17645053/226786119-d20063da-8763-41ce-9b00-ae34929120e1.mov

---------

Co-authored-by: silverwind <[email protected]>

Author: HesterG

package-lock.json

@@ -37,6 +37,7 @@
     "pretty-ms": "8.0.0",
     "sortablejs": "1.15.0",
     "swagger-ui-dist": "4.18.1",
+    "throttle-debounce": "5.0.0",
     "tippy.js": "6.3.7",
     "tributejs": "5.1.3",
     "uint8-to-base64": "0.2.0",

package.json

@@ -30,8 +30,8 @@
 			<div class="field">
 				<div class="ui top attached tabular menu" data-write="write" data-preview="preview" data-diff="diff">
 					<a class="active item" data-tab="write">{{svg "octicon-code"}} {{if .IsNewFile}}{{.locale.Tr "repo.editor.new_file"}}{{else}}{{.locale.Tr "repo.editor.edit_file"}}{{end}}</a>
-					{{if not .IsNewFile}}
 					<a class="item" data-tab="preview" data-url="{{.Repository.Link}}/markup" data-context="{{.RepoLink}}/src/{{.BranchNameSubURL}}" data-markup-mode="file">{{svg "octicon-eye"}} {{.locale.Tr "preview"}}</a>
+					{{if not .IsNewFile}}
 					<a class="item" data-tab="diff" data-url="{{.RepoLink}}/_preview/{{.BranchName | PathEscapeSegments}}/{{.TreePath | PathEscapeSegments}}" data-context="{{.BranchLink}}">{{svg "octicon-diff"}} {{.locale.Tr "repo.editor.preview_changes"}}</a>
 					{{end}}
 				</div>

templates/repo/editor/edit.tmpl

@@ -1,4 +1,5 @@
 import {basename, extname, isObject, isDarkTheme} from '../utils.js';
+import {debounce} from 'throttle-debounce';
 
 const languagesByFilename = {};
 const languagesByExt = {};
@@ -130,34 +131,46 @@ function getFileBasedOptions(filename, lineWrapExts) {
   };
 }
 
+function togglePreviewDisplay(previewable) {
+  const previewTab = document.querySelector('a[data-tab="preview"]');
+  if (!previewTab) return;
+
+  if (previewable) {
+    const newUrl = (previewTab.getAttribute('data-url') || '').replace(/(.*)\/.*/i, `$1/markup`);
+    previewTab.setAttribute('data-url', newUrl);
+    previewTab.style.display = '';
+  } else {
+    previewTab.style.display = 'none';
+    // If the "preview" tab was active, user changes the filename to a non-previewable one,
+    // then the "preview" tab becomes inactive (hidden), so the "write" tab should become active
+    if (previewTab.classList.contains('active')) {
+      const writeTab = document.querySelector('a[data-tab="write"]');
+      writeTab.click();
+    }
+  }
+}
+
 export async function createCodeEditor(textarea, filenameInput) {
   const filename = basename(filenameInput.value);
-  const previewLink = document.querySelector('a[data-tab=preview]');
-  const previewableExts = (textarea.getAttribute('data-previewable-extensions') || '').split(',');
+  const previewableExts = new Set((textarea.getAttribute('data-previewable-extensions') || '').split(','));
   const lineWrapExts = (textarea.getAttribute('data-line-wrap-extensions') || '').split(',');
-  const previewable = previewableExts.includes(extname(filename));
+  const previewable = previewableExts.has(extname(filename));
   const editorConfig = getEditorconfig(filenameInput);
 
-  if (previewLink) {
-    if (previewable) {
-      const newUrl = (previewLink.getAttribute('data-url') || '').replace(/(.*)\/.*/i, `$1/markup`);
-      previewLink.setAttribute('data-url', newUrl);
-      previewLink.style.display = '';
-    } else {
-      previewLink.style.display = 'none';
-    }
-  }
+  togglePreviewDisplay(previewable);
 
   const {monaco, editor} = await createMonaco(textarea, filename, {
     ...baseOptions,
     ...getFileBasedOptions(filenameInput.value, lineWrapExts),
     ...getEditorConfigOptions(editorConfig),
   });
 
-  filenameInput.addEventListener('keyup', () => {
+  filenameInput.addEventListener('input', debounce(500, () => {
     const filename = filenameInput.value;
+    const previewable = previewableExts.has(extname(filename));
+    togglePreviewDisplay(previewable);
     updateEditor(monaco, editor, filename, lineWrapExts);
-  });
+  }));
 
   return editor;
 }