ui : keep the model selector mounted across backend switches

Assisted-by: pi:llama.cpp/DeepSeek-V4.1-Flash

tools/ui/src/lib/components/app/models/ModelsSelector/ModelsSelectorDropdown.svelte

@@ -276,13 +276,6 @@
 					class="w-full md:min-w-80 md:max-w-[26rem] max-w-[calc(100vw-2rem)] p-0!"
 					onOpenAutoFocus={(event) => event.preventDefault()}
 				>
-					<ModelsSelectorBackendSwitcher
-						activeId={ms.activeBackendId}
-						backends={ms.backends}
-						onAdd={handleAddBackend}
-						onSelect={(backendId) => void ms.handleBackendChange(backendId)}
-					/>
-
 					<DropdownMenuSearchable
 						emptyMessage="No models found."
 						isEmpty={ms.filteredOptions.length === 0 && ms.isCurrentModelInCache}
@@ -292,6 +285,14 @@
 						searchClass="bg-transparent"
 						searchValue={ms.searchTerm}
 					>
+						<!-- Provider tabs sit under the search input, above the option list. -->
+						<ModelsSelectorBackendSwitcher
+							activeId={ms.activeBackendId}
+							backends={ms.backends}
+							onAdd={handleAddBackend}
+							onSelect={(backendId) => void ms.handleBackendChange(backendId)}
+						/>
+
 						<!-- Option list; the search header sticks to the top and the actions
 						     footer to the bottom of the content scrollport. -->
 						<div class="models-list px-1.5">

tools/ui/src/lib/components/app/models/ModelsSelector/ModelsSelectorSheet.svelte

@@ -91,7 +91,7 @@
 			? Math.round(modelLoadFraction(modelsStore.status.getLoadProgress(triggerModel)) * 100)
 			: 0}
 
-		{#if ms.isMultiModel}
+		{#if ms.isMultiModel || ms.switchingBackends}
 			<button
 				class={[
 					`relative inline-flex cursor-pointer items-center gap-1.5 rounded-sm bg-background px-1.5 py-1 text-xs shadow-sm transition hover:bg-muted-foreground/20 focus:outline-none focus-visible:ring-2 focus-visible:ring-ring focus-visible:ring-offset-2 disabled:cursor-not-allowed disabled:opacity-60 max-sm:px-3 max-sm:py-2 max-sm:text-sm dark:bg-muted-foreground/15 dark:text-secondary-foreground`,
@@ -127,7 +127,7 @@
 					<Lightbulb class="h-3.5 w-3.5 shrink-0 text-amber-400" />
 				{/if}
 
-				{#if ms.updating || ms.isLoadingModel}
+				{#if ms.updating || ms.isLoadingModel || ms.switchingBackends}
 					<Loader2 class="h-3 w-3.5 shrink-0 animate-spin" />
 				{:else}
 					<ChevronDown class="h-3 w-3.5 shrink-0" />
@@ -149,13 +149,6 @@
 					</Sheet.Header>
 
 					<div class="flex flex-col gap-1 pb-4">
-						<ModelsSelectorBackendSwitcher
-							activeId={ms.activeBackendId}
-							backends={ms.backends}
-							onAdd={handleAddBackend}
-							onSelect={(backendId) => void ms.handleBackendChange(backendId)}
-						/>
-
 						<div class="mb-3 px-4">
 							<SearchInput
 								onInput={(v) => ms.setSearchTerm(v)}
@@ -164,6 +157,14 @@
 							/>
 						</div>
 
+						<!-- Provider tabs sit under the search input, above the option list. -->
+						<ModelsSelectorBackendSwitcher
+							activeId={ms.activeBackendId}
+							backends={ms.backends}
+							onAdd={handleAddBackend}
+							onSelect={(backendId) => void ms.handleBackendChange(backendId)}
+						/>
+
 						<div class="max-h-[60vh] overflow-y-auto px-2">
 							{#if !ms.isCurrentModelInCache && currentModel}
 								<button

tools/ui/src/lib/hooks/use-models-selector.svelte.ts

@@ -30,6 +30,7 @@ export interface UseModelsSelectorReturn {
 	readonly filteredOptions: ModelOption[];
 	readonly groupedFilteredOptions: ReturnType<typeof groupModelOptions>;
 	readonly isLoadingModel: boolean;
+	readonly switchingBackends: boolean;
 	readonly searchTerm: string;
 	readonly showModelDialog: boolean;
 	readonly infoModelId: string | null;
@@ -88,6 +89,7 @@ export function useModelsSelector(opts: UseModelsSelectorOptions): UseModelsSele
 	});
 
 	let isLoadingModel = $state(false);
+	let switchingBackends = $state(false);
 	let searchTerm = $state('');
 	let showModelDialog = $state(false);
 	let infoModelId = $state<string | null>(null);
@@ -141,11 +143,18 @@ export function useModelsSelector(opts: UseModelsSelectorOptions): UseModelsSele
 		backendsStore.setActive(backendId);
 		searchTerm = '';
 
+		// keep the multi-model selector mounted while the new backend's props and
+		// models load; role flips (external MODEL mode -> local ROUTER mode) would
+		// otherwise unmount and remount the open dropdown mid switch
+		switchingBackends = true;
+
 		try {
 			await backendsModelsStore.ensureLoaded(backendId);
 			await modelsStore.switchBackend();
 		} catch (error) {
 			console.error('Failed to switch backend:', error);
+		} finally {
+			switchingBackends = false;
 		}
 	}
 
@@ -321,6 +330,10 @@ export function useModelsSelector(opts: UseModelsSelectorOptions): UseModelsSele
 			return showModelDialog;
 		},
 
+		get switchingBackends() {
+			return switchingBackends;
+		},
+
 		get updating() {
 			return updating;
 		}