fix(shell): update shell to correctly position calcite shell panel at shell's bottom (#9748)

**Related Issue:**
[#8269](#8269)

## Summary
Update `calcite-shell` to correctly position `calcite-shell-panel` with
`slot="panel-bottom"` at calcite-shell's bottom, when there is no
`calcite-shell-panel` with `slot="panel-top"`.

Author: aPreciado88

packages/calcite-components/src/components/shell/resources.ts

@@ -2,6 +2,7 @@ export const CSS = {
   main: "main",
   content: "content",
   contentBehind: "content--behind",
+  contentBottom: "content-bottom",
   contentNonInteractive: "content--non-interactive",
   footer: "footer",
   positionedSlotWrapper: "positioned-slot-wrapper",

packages/calcite-components/src/components/shell/shell.e2e.ts

@@ -121,4 +121,22 @@ describe("calcite-shell", () => {
     const panelTop = await contentNode.find(`slot[name="${SLOTS.panelTop}"]`);
     expect(panelTop).toBeNull();
   });
+
+  it("should position panel-bottom slot at content's bottom when no other panels exist", async () => {
+    const page = await newE2EPage();
+
+    await page.setContent(
+      html`<calcite-shell>
+        <calcite-shell-panel slot="${SLOTS.panelBottom}" display-mode="float" layout="horizontal">
+          <p>Primary Content</p>
+        </calcite-shell-panel>
+      </calcite-shell>`,
+    );
+
+    await page.waitForChanges();
+
+    const contentBottom = await page.find(`calcite-shell >>> .${CSS.contentBottom}`);
+
+    expect(contentBottom).not.toBeNull();
+  });
 });

packages/calcite-components/src/components/shell/shell.scss

@@ -40,6 +40,7 @@
 
 .content {
   @apply overflow-auto;
+  justify-content: space-between;
 }
 
 .content ::slotted(calcite-shell-center-row),
@@ -97,6 +98,10 @@ slot[name="panel-bottom"]::slotted(calcite-shell-center-row:not([detached])) {
   min-inline-size: 0;
 }
 
+.content-bottom {
+  justify-content: flex-end;
+}
+
 ::slotted(calcite-shell-center-row) {
   flex: none;
   align-self: stretch;

packages/calcite-components/src/components/shell/shell.stories.ts

@@ -99,6 +99,12 @@ const centerRowHTML = html`
   </calcite-panel>
 `;
 
+const bottomPanelHTML = html`
+  <calcite-panel heading="Panel bottom content">
+    <div>Content</div>
+  </calcite-panel>
+`;
+
 const centerRowWithActionBarHTML = html`
   <calcite-action-bar slot="action-bar">
     <calcite-action-group>
@@ -842,6 +848,30 @@ export const slottedPanelTop_TestOnly = (): string =>
   </calcite-shell>
 `);
 
+export const contentBehindPanelBottom = (): string =>
+  html(`
+  <calcite-shell
+    content-behind
+    style="
+    width:700px;
+    height:700px;
+    position:relative;
+    "
+  >
+      <div
+      style="
+      width:100%;
+      height:100%;
+      background-image: linear-gradient(45deg, #ccc 25%, transparent 25%),
+      linear-gradient(-45deg, #ccc 25%, transparent 25%),
+      linear-gradient(45deg, transparent 75%, #ccc 75%),
+      linear-gradient(-45deg, transparent 75%, #ccc 75%);
+      background-size: 20px 20px;
+      background-position: 0 0, 0 10px, 10px -10px, -10px 0px;"></div>
+      <calcite-shell-panel slot="panel-bottom" display-mode="float" layout="horizontal">${bottomPanelHTML}</calcite-shell-panel>
+    </calcite-shell>
+  `);
+
 export const slottedPanelBottom_TestOnly = (): string =>
   html(`
   <calcite-shell

packages/calcite-components/src/components/shell/shell.tsx

@@ -1,4 +1,4 @@
-import { Component, Element, Fragment, h, Listen, Prop, State, VNode } from "@stencil/core";
+import { Component, Element, Fragment, h, Listen, Prop, State, VNode, Watch } from "@stencil/core";
 import {
   ConditionalSlotComponent,
   connectConditionalSlotComponent,
@@ -74,8 +74,20 @@ export class Shell implements ConditionalSlotComponent {
 
   @State() hasSheets = false;
 
+  @State() hasPanelTop = false;
+
+  @State() hasPanelBottom = false;
+
+  @State() hasOnlyPanelBottom = false;
+
   @State() panelIsResizing = false;
 
+  @Watch("hasPanelTop")
+  @Watch("hasPanelBottom")
+  updateHasOnlyPanelBottom(): void {
+    this.hasOnlyPanelBottom = !this.hasPanelTop && this.hasPanelBottom;
+  }
+
   // --------------------------------------------------------------------------
   //
   //  Lifecycle
@@ -131,6 +143,14 @@ export class Shell implements ConditionalSlotComponent {
     });
   };
 
+  handlePanelTopChange = (event: Event): void => {
+    this.hasPanelTop = slotChangeHasAssignedElement(event);
+  };
+
+  handlePanelBottomChange = (event: Event): void => {
+    this.hasPanelBottom = slotChangeHasAssignedElement(event);
+  };
+
   // --------------------------------------------------------------------------
   //
   //  Render Methods
@@ -188,8 +208,16 @@ export class Shell implements ConditionalSlotComponent {
     const deprecatedCenterRowSlotNode: VNode = (
       <slot key="center-row-slot" name={SLOTS.centerRow} />
     );
-    const panelBottomSlotNode: VNode = <slot key="panel-bottom-slot" name={SLOTS.panelBottom} />;
-    const panelTopSlotNode: VNode = <slot key="panel-top-slot" name={SLOTS.panelTop} />;
+    const panelBottomSlotNode: VNode = (
+      <slot
+        key="panel-bottom-slot"
+        name={SLOTS.panelBottom}
+        onSlotchange={this.handlePanelBottomChange}
+      />
+    );
+    const panelTopSlotNode: VNode = (
+      <slot key="panel-top-slot" name={SLOTS.panelTop} onSlotchange={this.handlePanelTopChange} />
+    );
 
     const contentContainerKey = "content-container";
 
@@ -204,14 +232,22 @@ export class Shell implements ConditionalSlotComponent {
           >
             {defaultSlotContainerNode}
           </div>,
-          <div class={CSS.contentBehindCenterContent}>
+          <div
+            class={{
+              [CSS.contentBehindCenterContent]: true,
+              [CSS.contentBottom]: this.hasOnlyPanelBottom,
+            }}
+          >
             {panelTopSlotNode}
             {panelBottomSlotNode}
             {deprecatedCenterRowSlotNode}
           </div>,
         ]
       : [
-          <div class={CSS.content} key={contentContainerKey}>
+          <div
+            class={{ [CSS.content]: true, [CSS.contentBottom]: this.hasOnlyPanelBottom }}
+            key={contentContainerKey}
+          >
             {panelTopSlotNode}
             {defaultSlotContainerNode}
             {panelBottomSlotNode}