feat(ui): redesign context and compression UI for a more seamless experience

- Added ui.showContextWindowWarning setting (default false).
- Implemented forced auto-compression on context overflow when warning is disabled.
- Redesigned compression messages to be subtle (gray, no icon, left border).
- Removed automatic context usage percentage from the minimal/focus UI.
- Changed ui.hideContextSummary default to true.
- Updated and verified all relevant tests.

Note: Includes a behavioral change where the CLI now attempts to force-compress
history when the context window is full rather than blocking by default.

packages/cli/src/ui/components/messages/CompressionMessage.tsx

@@ -9,7 +9,7 @@ import type { CompressionProps } from '../../types.js';
 import { CliSpinner } from '../CliSpinner.js';
 import { theme } from '../../semantic-colors.js';
 import { SCREEN_READER_MODEL_PREFIX } from '../../textConstants.js';
-import { CompressionStatus } from '@google/gemini-cli-core';
+import { CompressionStatus, tokenLimit } from '@google/gemini-cli-core';
 
 export interface CompressionDisplayProps {
   compression: CompressionProps;
@@ -22,8 +22,13 @@ export interface CompressionDisplayProps {
 export function CompressionMessage({
   compression,
 }: CompressionDisplayProps): React.JSX.Element {
-  const { isPending, originalTokenCount, newTokenCount, compressionStatus } =
-    compression;
+  const {
+    isPending,
+    originalTokenCount,
+    newTokenCount,
+    compressionStatus,
+    model,
+  } = compression;
 
   const originalTokens = originalTokenCount ?? 0;
   const newTokens = newTokenCount ?? 0;
@@ -33,9 +38,15 @@ export function CompressionMessage({
       return 'Compressing chat history';
     }
 
+    const limit = model ? tokenLimit(model) : 0;
+    const formatPercent = (tokens: number) =>
+      limit > 0
+        ? `${Math.round((tokens / limit) * 100)}% (${tokens.toLocaleString()} tokens)`
+        : `${tokens.toLocaleString()} tokens`;
+
     switch (compressionStatus) {
       case CompressionStatus.COMPRESSED:
-        return `Chat history compressed from ${originalTokens} to ${newTokens} tokens.`;
+        return `Context compressed from ${formatPercent(originalTokens)} to ${formatPercent(newTokens)}. Change threshold in /settings.`;
       case CompressionStatus.COMPRESSION_FAILED_INFLATED_TOKEN_COUNT:
         // For smaller histories (< 50k tokens), compression overhead likely exceeds benefits
         if (originalTokens < 50000) {
@@ -43,11 +54,11 @@ export function CompressionMessage({
         }
         // For larger histories where compression should work but didn't,
         // this suggests an issue with the compression process itself
-        return 'Chat history compression did not reduce size. This may indicate issues with the compression prompt.';
+        return 'Chat history compression did not reduce size.';
       case CompressionStatus.COMPRESSION_FAILED_TOKEN_COUNT_ERROR:
         return 'Could not compress chat history due to a token counting error.';
       case CompressionStatus.COMPRESSION_FAILED_EMPTY_SUMMARY:
-        return 'Chat history compression failed: the model returned an empty summary.';
+        return 'Chat history compression failed: empty summary.';
       case CompressionStatus.NOOP:
         return 'Nothing to compress.';
       default:
@@ -58,20 +69,13 @@ export function CompressionMessage({
   const text = getCompressionText();
 
   return (
-    <Box flexDirection="row">
-      <Box marginRight={1}>
-        {isPending ? (
-          <CliSpinner type="dots" />
-        ) : (
-          <Text color={theme.text.accent}>✦</Text>
-        )}
-      </Box>
+    <Box flexDirection="row" paddingLeft={1} marginBottom={1}>
+      <Box marginRight={1}>{isPending && <CliSpinner type="dots" />}</Box>
       <Box>
         <Text
-          color={
-            compression.isPending ? theme.text.accent : theme.status.success
-          }
+          color={theme.text.secondary}
           aria-label={SCREEN_READER_MODEL_PREFIX}
+          italic
         >
           {text}
         </Text>