fix(terminal): inset the grid inside the xterm surface (#13252)

Padding X/Y was applied as start-edge container margin, so the cell grid
stayed flush on the trailing edges and a fractional background opacity
stacked a darker gutter around the viewport. Put the setting on .xterm
so FitAddon insets both axes and the themed background fills the pad.

src/renderer/src/assets/terminal-container-geometry.test.ts

@@ -5,9 +5,7 @@ const terminalCss = fs.readFileSync(new URL('./terminal.css', import.meta.url),
 
 describe('terminal container geometry', () => {
   it('keeps the hidden link tooltip out of the fitted terminal height', () => {
-    expect(terminalCss).toMatch(
-      /\.xterm-container\s*{[^}]*height:\s*calc\(100% - var\(--pane-padding-y, 4px\)\);/s
-    )
+    expect(terminalCss).toMatch(/\.xterm-container\s*{[^}]*height:\s*100%;/s)
     expect(terminalCss).toMatch(
       /\.pane\[data-has-title\] \.xterm-container\s*{[^}]*height:\s*calc\(100% - var\(--orca-pane-title-height\)\);/s
     )
@@ -15,4 +13,21 @@ describe('terminal container geometry', () => {
       /\.pane-link-tooltip\s*{[^}]*height:\s*var\(--orca-terminal-link-tooltip-height\);/s
     )
   })
+
+  it('insets the xterm grid on both axes from pane padding', () => {
+    expect(terminalCss).toMatch(
+      /\.xterm-container \.xterm\s*{[^}]*padding-top:\s*var\(--pane-padding-y, 4px\);/s
+    )
+    expect(terminalCss).toMatch(
+      /\.xterm-container \.xterm\s*{[^}]*padding-right:\s*var\(--pane-padding-x, 4px\);/s
+    )
+    expect(terminalCss).toMatch(
+      /\.xterm-container \.xterm\s*{[^}]*padding-bottom:\s*var\(--pane-padding-y, 4px\);/s
+    )
+    expect(terminalCss).toMatch(
+      /\.xterm-container \.xterm\s*{[^}]*padding-left:\s*var\(--pane-padding-x, 4px\);/s
+    )
+    expect(terminalCss).toMatch(/\.xterm-container\s*{[^}]*width:\s*100%;/s)
+    expect(terminalCss).not.toMatch(/\.xterm-container\s*{[^}]*margin-left:/s)
+  })
 })

src/renderer/src/assets/terminal.css

@@ -30,6 +30,20 @@
   width: 100%;
 }
 
+.xterm-container .xterm {
+  box-sizing: border-box;
+  height: 100%;
+  width: 100%;
+  /* Why on .xterm, not the container: FitAddon subtracts this element's
+     padding when proposing cols/rows, and the themed background (including
+     opacity) fills the inset. Container margins only framed the start edges
+     and stacked a darker gutter at fractional opacity. */
+  padding-top: var(--pane-padding-y, 4px);
+  padding-right: var(--pane-padding-x, 4px);
+  padding-bottom: var(--pane-padding-y, 4px);
+  padding-left: var(--pane-padding-x, 4px);
+}
+
 .pane-manager-root .xterm-viewport {
   /* Why: xterm/VS Code keep the scrollbar gutter stable. `auto` lets Linux
      DOM scrollbars appear/disappear, which can feed resize/reflow jitter. */
@@ -177,7 +191,7 @@
 .pane[data-terminal-attention] .xterm-container::after {
   content: '';
   position: absolute;
-  inset: 0 4px 4px 0;
+  inset: 0;
   z-index: 5;
   pointer-events: none;
   border-radius: 2px;
@@ -498,16 +512,14 @@
   border-color: var(--ring);
 }
 
-/* Baseline xterm-container layout — padding is driven by CSS custom properties
-   so Ghostty-imported window-padding-x/y can be applied without inline styles.
-   The fallback 4px preserves the legacy default. */
+/* Container fills the pane so the .xterm background (and its padding) is the
+   terminal surface. Title offset stays here because the title overlay is a
+   sibling of the container, not of .xterm. */
 .xterm-container {
   box-sizing: border-box;
   position: relative;
-  width: calc(100% - var(--pane-padding-x, 4px));
-  height: calc(100% - var(--pane-padding-y, 4px));
-  margin-top: var(--pane-padding-y, 4px);
-  margin-left: var(--pane-padding-x, 4px);
+  width: 100%;
+  height: 100%;
 }
 
 /* When a pane has a title, shift the terminal content down to make room.

src/renderer/src/components/terminal-pane/terminal-appearance.test.ts

@@ -360,6 +360,24 @@ describe('applyTerminalAppearance theme assignment', () => {
     // Latest wins, exactly one write: intermediate hidden values never touch xterm.
     expect(writes).toEqual([21])
   })
+
+  it('stamps settings-UI padding defaults when padding is unset', () => {
+    const pane = makePane(1)
+    const manager = makeManager([pane])
+    applyTerminalAppearance(
+      manager,
+      getDefaultSettings('/tmp'),
+      true,
+      new Map(),
+      new Map(),
+      'false',
+      new Map(),
+      new Map()
+    )
+    expect(manager.setPaneStyleOptions).toHaveBeenCalledWith(
+      expect.objectContaining({ paddingX: 4, paddingY: 4 })
+    )
+  })
 })
 
 describe('publishTerminalViewAttributesAtAppStart', () => {

src/renderer/src/components/terminal-pane/terminal-appearance.ts

@@ -159,6 +159,20 @@ export function applyTerminalAppearance(
     settings.terminalFontFamily
   )
 
+  // Why before the pane loop: FitAddon subtracts live .xterm padding, so the
+  // CSS vars must be stamped before safeFit or a padding shrink leaves a stale grid.
+  manager.setPaneStyleOptions({
+    splitBackground: paneBackground,
+    paneBackground,
+    inactivePaneOpacity: paneStyles.inactivePaneOpacity,
+    activePaneOpacity: paneStyles.activePaneOpacity,
+    opacityTransitionMs: paneStyles.opacityTransitionMs,
+    dividerThicknessPx: paneStyles.dividerThicknessPx,
+    focusFollowsMouse: paneStyles.focusFollowsMouse,
+    paddingX: settings.terminalPaddingX ?? 4,
+    paddingY: settings.terminalPaddingY ?? 4
+  })
+
   for (const pane of manager.getPanes()) {
     // Why value-gated: writing options.theme rebuilds the palette, discarding TUI OSC 4/10/11/12 mutations; skip on no-op change.
     if (theme && !composedTerminalThemesEqual(pane.terminal.options.theme, theme)) {
@@ -227,16 +241,4 @@ export function applyTerminalAppearance(
       safeFit(pane)
     }
   }
-
-  manager.setPaneStyleOptions({
-    splitBackground: paneBackground,
-    paneBackground,
-    inactivePaneOpacity: paneStyles.inactivePaneOpacity,
-    activePaneOpacity: paneStyles.activePaneOpacity,
-    opacityTransitionMs: paneStyles.opacityTransitionMs,
-    dividerThicknessPx: paneStyles.dividerThicknessPx,
-    focusFollowsMouse: paneStyles.focusFollowsMouse,
-    paddingX: settings.terminalPaddingX,
-    paddingY: settings.terminalPaddingY
-  })
 }