diff --git a/package-lock.json b/package-lock.json
index 8884150aba3..ce30af175a5 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -2134,9 +2134,9 @@
       "integrity": "sha512-9nWywp+0SH7ROVzQPQQO9gMWBikahsqyMWp1Ku8VV0q+q6bnx6dS0aNPTjqTtF2GHAY55hcREsqKzaoUdWBSwg=="
     },
     "node_modules/@turbowarp/jszip": {
-      "version": "3.11.0",
-      "resolved": "https://registry.npmjs.org/@turbowarp/jszip/-/jszip-3.11.0.tgz",
-      "integrity": "sha512-G+xld2rPv0b7ZKIk4ZZLW8l46+VfdUsreYjWml8bl3IHrmQXgijkmiDwTq17CBf5GPlz4+F/TpJdclq0jhMSVQ==",
+      "version": "3.11.1",
+      "resolved": "https://registry.npmjs.org/@turbowarp/jszip/-/jszip-3.11.1.tgz",
+      "integrity": "sha512-1tWXTxAac1T/g0VHC9lIY0Ij7Qyt7sORIaAT4L0/Y+pjU1ZtXD9ti/+RnXzTVHXp6AM8fM2O3mF22/aSEVPXiQ==",
       "license": "(MIT OR GPL-3.0-or-later)",
       "dependencies": {
         "lie": "~3.3.0",
@@ -2187,12 +2187,12 @@
       "integrity": "sha512-texcM9oxfEsADVlVDR5UhLkYclPKsV9mytJh+9pHHonNcUrxRVGF6FkJTzWO/Hl5NafU1crSdw737nqKE3atSA=="
     },
     "node_modules/@turbowarp/sb3fix": {
-      "version": "0.3.0",
-      "resolved": "https://registry.npmjs.org/@turbowarp/sb3fix/-/sb3fix-0.3.0.tgz",
-      "integrity": "sha512-tZjpPb37UUnr6mxeZ12FLqHFhQ66rIUEIfFMWoLLzmt8VFE6JjM74chyphsB3AwON5ehs678zzXkhRSOZgPK3A==",
+      "version": "0.3.6",
+      "resolved": "https://registry.npmjs.org/@turbowarp/sb3fix/-/sb3fix-0.3.6.tgz",
+      "integrity": "sha512-pKRyJbZYGHFAQNacwJEoz5sh8SnSDRFFle1BcEoyZHRuDE/9TZoBFNfYzoyjKCeOyOBStcS8kmGhak45YabaDg==",
       "license": "MPL-2.0",
       "dependencies": {
-        "@turbowarp/jszip": "^3.11.0"
+        "@turbowarp/jszip": "^3.11.1"
       }
     },
     "node_modules/@turbowarp/scratch-svg-renderer": {
diff --git a/src/io/mouse.js b/src/io/mouse.js
index 169e6af0b67..bc46d907d84 100644
--- a/src/io/mouse.js
+++ b/src/io/mouse.js
@@ -101,16 +101,19 @@ class Mouse {
             if (!(data.x > 0 && data.x < data.canvasWidth &&
                 data.y > 0 && data.y < data.canvasHeight)) return;
 
+            // target will not exist if project is still loading
             const target = this._pickTarget(data.x, data.y);
-            const isNewMouseDown = !previousDownState && this._isDown;
-            const isNewMouseUp = previousDownState && !this._isDown;
-
-            // Draggable targets start click hats on mouse up.
-            // Non-draggable targets start click hats on mouse down.
-            if (target.draggable && isNewMouseUp) {
-                this._activateClickHats(target);
-            } else if (!target.draggable && isNewMouseDown) {
-                this._activateClickHats(target);
+            if (target) {
+                const isNewMouseDown = !previousDownState && this._isDown;
+                const isNewMouseUp = previousDownState && !this._isDown;
+
+                // Draggable targets start click hats on mouse up.
+                // Non-draggable targets start click hats on mouse down.
+                if (target.draggable && isNewMouseUp) {
+                    this._activateClickHats(target);
+                } else if (!target.draggable && isNewMouseDown) {
+                    this._activateClickHats(target);
+                }
             }
         }
     }
diff --git a/src/virtual-machine.js b/src/virtual-machine.js
index e02d87222a2..14ff8310a2f 100644
--- a/src/virtual-machine.js
+++ b/src/virtual-machine.js
@@ -537,6 +537,22 @@ class VirtualMachine extends EventEmitter {
             file.date = date;
         }
 
+        // Tell JSZip to only compress file formats where there will be a significant gain.
+        const COMPRESSABLE_FORMATS = [
+            '.json',
+            '.svg',
+            '.wav',
+            '.ttf',
+            '.otf'
+        ];
+        for (const file of Object.values(zip.files)) {
+            if (COMPRESSABLE_FORMATS.some(ext => file.name.endsWith(ext))) {
+                file.options.compression = 'DEFLATE';
+            } else {
+                file.options.compression = 'STORE';
+            }
+        }
+
         return zip;
     }
 
@@ -546,9 +562,9 @@ class VirtualMachine extends EventEmitter {
      */
     saveProjectSb3 (type) {
         return this._saveProjectZip().generateAsync({
+            // Don't configure compression here. _saveProjectZip() will set it for each file.
             type: type || 'blob',
-            mimeType: 'application/x.scratch.sb3',
-            compression: 'DEFLATE'
+            mimeType: 'application/x.scratch.sb3'
         });
     }
 
diff --git a/test/fixtures/tw-mixed-file-formats.sb3 b/test/fixtures/tw-mixed-file-formats.sb3
new file mode 100644
index 00000000000..f06c40d680b
Binary files /dev/null and b/test/fixtures/tw-mixed-file-formats.sb3 differ
diff --git a/test/integration/tw_compression_per_file_type.js b/test/integration/tw_compression_per_file_type.js
new file mode 100644
index 00000000000..35f02a68cdb
--- /dev/null
+++ b/test/integration/tw_compression_per_file_type.js
@@ -0,0 +1,31 @@
+const {test} = require('tap');
+const fs = require('fs');
+const path = require('path');
+const VM = require('../../src/virtual-machine');
+const makeTestStorage = require('../fixtures/make-test-storage');
+const JSZip = require('@turbowarp/jszip');
+
+test('saveProjectSb3() per-file compression', t => {
+    const vm = new VM();
+    vm.attachStorage(makeTestStorage());
+    const fixture = fs.readFileSync(path.join(__dirname, '../fixtures/tw-mixed-file-formats.sb3'));
+    vm.loadProject(fixture)
+        .then(() => vm.saveProjectSb3('arraybuffer'))
+        .then(buffer => JSZip.loadAsync(buffer))
+        .then(zip => {
+            const isCompressed = pathInZip => {
+                // note: uses JSZip private APIs, not very stable, but it should be okay...
+                const file = zip.files[pathInZip];
+                return file._data.compression.magic === '\x08\x00';
+            };
+
+            t.ok(isCompressed('project.json'));
+            t.ok(isCompressed('5cb46ddd903fc2c9976ff881df9273c9.wav'));
+            t.ok(isCompressed('cd21514d0531fdffb22204e0ec5ed84a.svg'));
+
+            t.notOk(isCompressed('0b2e50ca4107ce57416e2ceb840a6347.jpg'));
+            t.notOk(isCompressed('5c8826d846c06dddeb77590e8792fb7d.png'));
+
+            t.end();
+        });
+});
diff --git a/test/unit/tw_mouse.js b/test/unit/tw_mouse.js
new file mode 100644
index 00000000000..17aeb200f37
--- /dev/null
+++ b/test/unit/tw_mouse.js
@@ -0,0 +1,14 @@
+const {test} = require('tap');
+const Runtime = require('../../src/engine/runtime');
+
+test('isDown does not error before project loads', t => {
+    const rt = new Runtime();
+    rt.ioDevices.mouse.postData({
+        isDown: true,
+        x: 20,
+        y: 20,
+        canvasWidth: 100,
+        canvasHeight: 100
+    });
+    t.end();
+});