Parts in black on the side of a resized image

Good morning,
I have a problem with ShotCut that I cannot resolve even though I have consulted several tutorials. Can you help me. I’ll give you a link to the recording of my manipulations. To summarize, I’m trying to put a white mask on certain parts of my video, in a corner of the image. So I created a small white rectangle in Photoshop with the appropriate pixel dimensions (actually an unnecessary step) and saved it in PNG format. I imported it into the playlist. I created a video track into which I dropped the image. The image occupies almost the entire image format but as it is in more square proportions, it leaves parts in black on the side. The problem already appears there. Why are the laterally unoccupied parts filled with an opaque black background? Afterwards when I put the dimension and position filter, my white rectangle is well cropped and moved but around it remains filled with opaque black. In this same filter, the background color is black with an alpha channel of zero.
I don’t understand !
A link to a screenshot of what I explained to you:!AkePtX1AyI_qj_Z2ZkQt5xLYi1lJjg?e=EiKUY6

Thanks in advance !

Hello @Francois_Brun. In the color window, set the alpha channel of the background color to zero (transparency). Sometimes doesn’t work the 1st time. Try again.
You could have created the white rectangle directly in Shotcut: tab Open Other → Color → White, validate and add to playlist, then on track 2.
Bonjour @Francois_Brun. Dans la fenêtre des couleurs, mettez le canal alpha de la couleur de fond à zéro (transparence). Quelquefois, ne fonctionne pas la 1ère fois. Recommencez.
Vous auriez pu créer directement le rectangle blanc dans Shotcut: Onglet Ouvrir Autre → Couleur (mal traduit en Français par Étalonnage) → Blanc, valider et ajouter à la liste de lecture, puis sur la piste 2

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J’ai suivi la deuxième méthode, et toujours le même résultat
I followed the second method and same résult again ! I show you the result with thi image :
Capture d’écran SHOTCUT 2023-10-31.png

Dans votre dernière capture d’écran, la couleur du fond du filtre Taille, Position & Rotation est encore sur NOIR (#000000) avec Alpha à 255. Changez la valeur Alpha pour 0 (zéro).
Alpha 255 = opaque
Alpha 0 = Transparent.
Voici ce que donne la deuxième méthode proposée par @SergeC:

In your last screenshot, the background color of the Size, Position & Rotate filter is still set to BLACK (#000000) with Alpha at 255. Change the Alpha value to 0 (zero).
Alpha 255 = Opaque
Alpha 0 = Transparent.

Here’s how the second method proposed by @SergeC works:

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Changez la valeur du canal alpha et validez OK. Elle doit être à 0 pour la transparence, comme indiqué par @MusicalBox.
Change the alpha channel value and OK. It should be 0 for transparency, as indicated by @MusicalBox.