Set scroll position at top of the textarea when opening the subtitle editor.

## Description

This set the position of the scrollbar at the top of the textarea when opening the __subtitle editor__.
Previously the textarea scroll position was at the bottom of the textarea which doesn't make much sense when you want to edit a subtitle : you most likely want to edit the beginning of the subtitle first.

This also set the caret position on the first character.

## Design decision

I had to use a *component approach* instead of an `<ng-template>` for the edition modal because the `@viewChild` directive doesn't work for elements __inside__ an `<ng-template>`.
I needed the `viewChild` directive to get an `ElementRef` of the `textarea`.

> See the following issue and its workaround :
> - https://github.com/valor-software/ngx-bootstrap/issues/3825
> - https://stackblitz.com/edit/angular-t5dfp7
> - https://medium.com/@izzatnadiri/how-to-pass-data-to-and-receive-from-ng-bootstrap-modals-916f2ad5d66e

## Related issues
Closes [peertube-plugin-transcription/#39](https://gitlab.com/apps_education/peertube/plugin-transcription/-/issues/39)

1 files changed, 91 insertions, 0 deletions

diff --git a/client/src/app/+videos/+video-edit/shared/video-caption-edit-modal-content/video-caption-edit-modal-content.component.ts b/client/src/app/+videos/+video-edit/shared/video-caption-edit-modal-content/video-caption-edit-modal-content.component.ts
new file mode 100644
index 000000000..f33353d36
--- /dev/null
+++ b/client/src/app/+videos/+video-edit/shared/video-caption-edit-modal-content/video-caption-edit-modal-content.component.ts
@@ -0,0 +1,91 @@
+import { Component, ElementRef, EventEmitter, Input, OnInit, Output, ViewChild } from '@angular/core'
+import { VIDEO_CAPTION_FILE_CONTENT_VALIDATOR } from '@app/shared/form-validators/video-captions-validators'
+import { FormReactive, FormValidatorService } from '@app/shared/shared-forms'
+import { VideoCaptionEdit, VideoCaptionService, VideoCaptionWithPathEdit } from '@app/shared/shared-main'
+import { NgbModal, NgbActiveModal } from '@ng-bootstrap/ng-bootstrap'
+import { HTMLServerConfig, VideoConstant } from '@shared/models'
+import { ServerService } from '../../../../core'
+
+/**
+ * https://github.com/valor-software/ngx-bootstrap/issues/3825
+ * https://stackblitz.com/edit/angular-t5dfp7
+ * https://medium.com/@izzatnadiri/how-to-pass-data-to-and-receive-from-ng-bootstrap-modals-916f2ad5d66e
+ */
+@Component({
+  selector: 'my-video-caption-edit-modal-content',
+  styleUrls: [ './video-caption-edit-modal-content.component.scss' ],
+  templateUrl: './video-caption-edit-modal-content.component.html'
+})
+
+export class VideoCaptionEditModalContentComponent extends FormReactive implements OnInit {
+  @Input() videoCaption: VideoCaptionWithPathEdit
+  @Input() serverConfig: HTMLServerConfig
+
+  @Output() captionEdited = new EventEmitter<VideoCaptionEdit>()
+
+  @ViewChild('textarea', { static: true }) textarea!: ElementRef
+
+  videoCaptionLanguages: VideoConstant<string>[] = []
+
+  constructor (
+    protected openedModal: NgbActiveModal,
+    protected formValidatorService: FormValidatorService,
+    private modalService: NgbModal,
+    private videoCaptionService: VideoCaptionService,
+    private serverService: ServerService
+  ) {
+    super()
+  }
+
+  ngOnInit () {
+    this.serverService.getVideoLanguages().subscribe(languages => this.videoCaptionLanguages = languages)
+
+    this.buildForm({ captionFileContent: VIDEO_CAPTION_FILE_CONTENT_VALIDATOR })
+
+    this.loadCaptionContent()
+  }
+
+  loadCaptionContent () {
+    const { captionPath } = this.videoCaption
+    if (!captionPath) return
+
+    this.videoCaptionService.getCaptionContent({ captionPath })
+      .subscribe(res => {
+        this.form.patchValue({
+          captionFileContent: res
+        })
+        this.resetTextarea()
+      })
+  }
+
+  resetTextarea () {
+    this.textarea.nativeElement.scrollTop = 0
+    this.textarea.nativeElement.selectionStart = 0
+    this.textarea.nativeElement.selectionEnd = 0
+  }
+
+  hide () {
+    this.openedModal.close()
+  }
+
+  cancel () {
+    this.hide()
+  }
+
+  updateCaption () {
+    const format = 'vtt'
+    const languageId = this.videoCaption.language.id
+    const languageObject = this.videoCaptionLanguages.find(l => l.id === languageId)
+
+    this.captionEdited.emit({
+      language: languageObject,
+      captionfile: new File([ this.form.value['captionFileContent'] ], `${languageId}.${format}`, {
+        type: 'text/vtt',
+        lastModified: Date.now()
+      }),
+      action: 'UPDATE'
+    })
+
+    this.hide()
+  }
+}