Implement phased dynamic screen layout sizing in cvd monitor

When running cvd monitor (or cvd start --daemon=false), the UI layout
dynamically apportions a constant 30-line content budget based on exactly
which runtime logs are actively open and populated:
- Phase 1 (Only assemble log ready): Takes 100% of vertical content space.
- Phase 2 (Only launcher log ready): Takes 100% of vertical content space.
- Phase 3 (launcher and kernel logs ready): launcher log takes the top 1/3,
  and kernel log takes the bottom 2/3 of vertical content space.
- Phase 4 (All 3 logs fully populated): When logcat starts writing messages,
  all three logs are displayed equally (10 lines each) across the screen.

Assisted-by: Jetski:gemini-2.5-pro
Bug: b/519304405
TAG=agy
CONV=34ddc7b8-a695-4937-be99-b796087dd987

Author: Databean

base/cvd/cuttlefish/host/commands/cvd/cli/commands/monitor/display.cc

@@ -99,10 +99,15 @@ LogMonitorDisplay::LogMonitorDisplay(size_t width)
       total_lines_drawn_(0),
       colorize_(ShouldColorizeOutput(1)) {}
 
-void LogMonitorDisplay::DrawFile(SharedFD fd, const std::string& title) {
+void LogMonitorDisplay::DrawFile(SharedFD fd, const std::string& title,
+                                 size_t max_lines) {
+  if (max_lines == 0) {
+    return;
+  }
   std::vector<std::string> lines;
   if (fd->IsOpen()) {
-    Result<std::vector<std::string>> lines_result = GetLastNLines(fd, 10);
+    Result<std::vector<std::string>> lines_result =
+        GetLastNLines(fd, max_lines);
     if (lines_result.ok()) {
       lines = *lines_result;
     } else {
@@ -120,7 +125,10 @@ void LogMonitorDisplay::DrawFile(SharedFD fd, const std::string& title) {
     lines.push_back(absl::StrCat("Error: ", fd->StrError()));
   }
 
-  while (lines.size() < 10) {
+  if (lines.size() > max_lines) {
+    lines.resize(max_lines);
+  }
+  while (lines.size() < max_lines) {
     lines.push_back("");
   }
 

base/cvd/cuttlefish/host/commands/cvd/cli/commands/monitor/display.h

@@ -66,7 +66,7 @@ class LogMonitorDisplay {
    * within a bordered box titled with the provided name.
    * If the file cannot be read, an error box is drawn instead.
    */
-  void DrawFile(SharedFD fd, const std::string& title);
+  void DrawFile(SharedFD fd, const std::string& title, size_t max_lines = 10);
 
   /**
    * Appends the final bottom border to the display and returns the

base/cvd/cuttlefish/host/commands/cvd/cli/commands/monitor/monitor.cc

@@ -18,6 +18,7 @@
 
 #include <errno.h>
 #include <fcntl.h>
+#include <stdio.h>
 #include <sys/eventfd.h>
 #include <sys/inotify.h>
 #include <sys/poll.h>
@@ -139,10 +140,29 @@ Result<void> MonitorLogs(const LocalInstance& instance, SharedFD stop_eventfd) {
     }
     LogMonitorDisplay display(width);
 
-    display.DrawFile(launcher_fd, using_assemble_log ? kLogNameAssembleCvd
-                                                     : kLogNameLauncher);
-    display.DrawFile(kernel_fd, kLogNameKernel);
-    display.DrawFile(logcat_fd, kLogNameLogcat);
+    bool logcat_ready = false;
+    if (logcat_fd->IsOpen() && logcat_fd->LSeek(0, SEEK_END) > 0) {
+      logcat_ready = true;
+    }
+
+    size_t total_content = 30;
+
+    const std::string launcher_name =
+        using_assemble_log ? kLogNameAssembleCvd : kLogNameLauncher;
+    size_t launcher_lines = total_content;
+    size_t kernel_lines = 0;
+    size_t logcat_lines = 0;
+    if (kernel_fd->IsOpen()) {
+      launcher_lines = total_content / 3;
+      kernel_lines = total_content - launcher_lines;
+    }
+    if (logcat_ready) {
+      kernel_lines = total_content / 3;
+      logcat_lines = total_content - launcher_lines - kernel_lines;
+    }
+    display.DrawFile(launcher_fd, launcher_name, launcher_lines);
+    display.DrawFile(kernel_fd, kLogNameKernel, kernel_lines);
+    display.DrawFile(logcat_fd, kLogNameLogcat, logcat_lines);
 
     const auto [output, total_lines_drawn] = display.Finalize();
     std::cout << output << std::flush;
@@ -171,7 +191,7 @@ Result<void> MonitorLogs(const LocalInstance& instance, SharedFD stop_eventfd) {
     // a fallback timeout to awake and retry.
     int timeout_ms = -1;
     if (dir_watch == -1 || kernel_watch == -1 || launcher_watch == -1 ||
-        logcat_watch == -1 || using_assemble_log) {
+        logcat_watch == -1 || !logcat_ready || using_assemble_log) {
       timeout_ms = 200;
     }