Don't autosize textarea in diff view (#26233) (#26244)

Backport #26233 by @silverwind

Resizing the comment editor can be a very expensive operation because it
triggers page reflows, which on large PRs can take upwards of seconds to
complete. Disable this mechanism on the diff page only where we know
that the page can get large.

Fixes #26201 for the textarea
editor.

I don't think this can be fixed for EasyMDE because as far as I can
tell, it exposes no option to disable this resizing.

Co-authored-by: silverwind <me@silverwind.io>

templates/repo/diff/comment_form.tmpl

@@ -17,6 +17,7 @@
 			"TextareaName" "content"
 			"TextareaPlaceholder" ($.locale.Tr "repo.diff.comment.placeholder")
 			"DropzoneParentContainer" "form"
+			"DisableAutosize" "true"
 		)}}
 
 		<div class="field footer gt-mx-3">

templates/shared/combomarkdowneditor.tmpl

@@ -10,6 +10,7 @@ Template Attributes:
 * TextareaPlaceholder: placeholder attribute for the textarea
 * TextareaAriaLabel: aria-label attribute for the textarea
 * DropzoneParentContainer: container for file upload (leave it empty if no upload)
+* DisableAutosize: whether to disable automatic height resizing
 */}}
 <div {{if .ContainerId}}id="{{.ContainerId}}"{{end}} class="combo-markdown-editor {{.ContainerClasses}}" data-dropzone-parent-container="{{.DropzoneParentContainer}}">
 	{{if .MarkdownPreviewUrl}}
@@ -45,7 +46,7 @@ Template Attributes:
 			</div>
 		</markdown-toolbar>
 		<text-expander keys=": @" suffix="">
-			<textarea class="markdown-text-editor js-quick-submit"{{if .TextareaName}} name="{{.TextareaName}}"{{end}}{{if .TextareaPlaceholder}} placeholder="{{.TextareaPlaceholder}}"{{end}}{{if .TextareaAriaLabel}} aria-label="{{.TextareaAriaLabel}}"{{end}}>{{.TextareaContent}}</textarea>
+			<textarea class="markdown-text-editor js-quick-submit"{{if .TextareaName}} name="{{.TextareaName}}"{{end}}{{if .TextareaPlaceholder}} placeholder="{{.TextareaPlaceholder}}"{{end}}{{if .TextareaAriaLabel}} aria-label="{{.TextareaAriaLabel}}"{{end}}{{if .DisableAutosize}} data-disable-autosize="{{.DisableAutosize}}"{{end}}>{{.TextareaContent}}</textarea>
 		</text-expander>
 		<script>
 			if (localStorage?.getItem('markdown-editor-monospace') === 'true') {

web_src/js/features/comp/ComboMarkdownEditor.js

@@ -68,7 +68,10 @@ class ComboMarkdownEditor {
     this.textarea.id = `_combo_markdown_editor_${String(elementIdCounter++)}`;
     this.textarea.addEventListener('input', (e) => this.options?.onContentChanged?.(this, e));
     this.applyEditorHeights(this.textarea, this.options.editorHeights);
-    this.textareaAutosize = autosize(this.textarea, {viewportMarginBottom: 130});
+
+    if (this.textarea.getAttribute('data-disable-autosize') !== 'true') {
+      this.textareaAutosize = autosize(this.textarea, {viewportMarginBottom: 130});
+    }
 
     this.textareaMarkdownToolbar = this.container.querySelector('markdown-toolbar');
     this.textareaMarkdownToolbar.setAttribute('for', this.textarea.id);
@@ -246,7 +249,7 @@ class ComboMarkdownEditor {
     } else {
       this.textarea.value = v;
     }
-    this.textareaAutosize.resizeToFit();
+    this.textareaAutosize?.resizeToFit();
   }
 
   focus() {