100% height issue

A practical, modern way to solve this is to treat the nav as a fixed-height top bar and make the SWF container fill the remaining viewport. That combines the useful parts of answers here: @PsychicTide’s idea to use the window height, @kvprajapati’s iframe suggestion (same layout rules apply), and @almostbob’s z-index layering — but do it with a robust layout so the Flash controls never drop below the browser viewport. Also set the SWF wmode to opaque/transparent so HTML can sit above the plugin.

Example (CSS + JS fallback):

html, body { height:100%; margin:0; }
nav {
  position: fixed;
  top: 0;
  left: 0;
  right: 0;
  height: 60px; /* replace with your nav height */
  z-index: 9999;
}
#flash-wrap {
  position: fixed;
  top: 60px; /* same as nav height */
  right: 0;
  bottom: 0;
  left: 0;
}
#flash-wrap object, #flash-wrap embed { width:100%; height:100%; }

function resizeFlash(){
  var navH = document.querySelector('nav').offsetHeight || 0;
  document.getElementById('flash-wrap').style.height = (window.innerHeight - navH) + 'px';
}
window.addEventListener('resize', resizeFlash);
resizeFlash();

Notes and gotchas: percent heights tied to body can be flaky (as @MidiMagic pointed out); prefer 100vh/calc(100vh - Xpx) or the resize JS fallback for broad compatibility. Make sure to remove default body margins and to set the SWF wmode param so the nav actually appears above Flash. Finally, Flash is end-of-life in modern browsers — consider replacing the SWF with HTML5 alternatives or an emulator like Ruffle (see Adobe’s Flash EOL and MDN docs on calc()/viewport units for details).