add drop-to-replace functionality for image prompt drop zone

src/wwwroot/css/genpage.css

@@ -1079,6 +1079,7 @@ body {
     position: relative;
     background-color: color-mix(in srgb, transparent 80%, var(--background));
     transition: background-color 0.2s ease-out;
+    margin: 0 2px;
 }
 .alt-prompt-image {
     max-height: 128px;
@@ -1115,6 +1116,10 @@ body {
     border: 1px solid black;
     cursor: pointer;
 }
+.alt-prompt-image-container.image-drop-replace-target .alt-prompt-image {
+    outline: 4px solid var(--emphasis);
+    border-radius: 5px;
+}
 .image-clear-button {
     vertical-align: middle;
     display: inline-block;

src/wwwroot/js/genpage/main.js

@@ -524,30 +524,70 @@ function autoRevealRevision() {
     }
 }
 
+let promptImageReplaceTarget = null;
+
+function setPromptImageReplaceTarget(target) {
+    if (promptImageReplaceTarget) {
+        promptImageReplaceTarget.classList.remove('image-drop-replace-target');
+    }
+    promptImageReplaceTarget = target;
+    if (promptImageReplaceTarget) {
+        promptImageReplaceTarget.classList.add('image-drop-replace-target');
+    }
+}
+
+function getPromptImageDropReplaceTarget(e) {
+    if (uiImprover.getFileList(e.dataTransfer, e).length == 0) {
+        return null;
+    }
+    let target = e.target.closest('.alt-prompt-image-container');
+    if (!target || !target.querySelector('.alt-prompt-image')) {
+        return null;
+    }
+    return target;
+}
+
 function imagePromptAddImage(file) {
-    let clearButton = getRequiredElementById('alt_prompt_image_clear_button');
-    let promptImageArea = getRequiredElementById('alt_prompt_image_area');
+    let replaceTarget = promptImageReplaceTarget;
+    setPromptImageReplaceTarget(null);
+    let existingImage = replaceTarget ? replaceTarget.querySelector('.alt-prompt-image') : null;
+    if (replaceTarget && !existingImage) {
+        replaceTarget = null;
+    }
     let reader = new FileReader();
     reader.onload = (e) => {
         let data = e.target.result;
-        let imageContainer = createDiv(null, 'alt-prompt-image-container');
-        let imageRemoveButton = createSpan(null, 'alt-prompt-image-container-remove-button', '×');
-        imageRemoveButton.addEventListener('click', (e) => {
-            imageContainer.remove();
-            autoRevealRevision();
-            genTabLayout.altPromptSizeHandle();
-        });
-        imageRemoveButton.title = 'Remove this image';
-        imageContainer.appendChild(imageRemoveButton);
-        let imageObject = new Image();
-        imageObject.src = data;
-        imageObject.height = 128;
-        imageObject.className = 'alt-prompt-image';
-        imageObject.dataset.filedata = data;
-        imageContainer.appendChild(imageObject);
+        if (replaceTarget && !replaceTarget.isConnected) {
+            imagePromptAddImage(file);
+            return;
+        }
+        if (existingImage) {
+            existingImage.src = data;
+            existingImage.height = 128;
+            existingImage.dataset.filedata = data;
+        }
+        else {
+            let promptImageArea = getRequiredElementById('alt_prompt_image_area');
+            let imageContainer = createDiv(null, 'alt-prompt-image-container');
+            let imageRemoveButton = createSpan(null, 'alt-prompt-image-container-remove-button', '×');
+            imageRemoveButton.addEventListener('click', () => {
+                imageContainer.remove();
+                autoRevealRevision();
+                genTabLayout.altPromptSizeHandle();
+            });
+            imageRemoveButton.title = 'Remove this image';
+            imageContainer.appendChild(imageRemoveButton);
+            let imageObject = new Image();
+            imageObject.src = data;
+            imageObject.height = 128;
+            imageObject.className = 'alt-prompt-image';
+            imageObject.dataset.filedata = data;
+            imageContainer.appendChild(imageObject);
+            promptImageArea.appendChild(imageContainer);
+        }
+        let clearButton = getRequiredElementById('alt_prompt_image_clear_button');
         clearButton.style.display = '';
         showRevisionInputs(true);
-        promptImageArea.appendChild(imageContainer);
         genTabLayout.altPromptSizeHandle();
     };
     reader.readAsDataURL(file);
@@ -574,6 +614,25 @@ function imagePromptInputHandler() {
             }
         }
     });
+    let updateReplaceTarget = (e) => {
+        setPromptImageReplaceTarget(getPromptImageDropReplaceTarget(e));
+    };
+    dragArea.addEventListener('dragenter', updateReplaceTarget, true);
+    dragArea.addEventListener('dragover', updateReplaceTarget, true);
+    dragArea.addEventListener('dragleave', (e) => {
+        if (!dragArea.contains(e.relatedTarget)) {
+            setPromptImageReplaceTarget(null);
+        }
+    }, true);
+    dragArea.addEventListener('drop', (e) => {
+        setPromptImageReplaceTarget(getPromptImageDropReplaceTarget(e));
+    }, true);
+    document.addEventListener('drop', () => {
+        setPromptImageReplaceTarget(null);
+    }, true);
+    document.addEventListener('dragend', () => {
+        setPromptImageReplaceTarget(null);
+    }, true);
 }
 imagePromptInputHandler();